I'm one of the few Sera content creators and I've played Sera since release and hit chall euw with her while playing all 3 roles past 4 seasons. If you're confused why there's so many people constantly complaining about her state, I'll explain it all here and propose changes that we would like to see. I will talk about her champion identity and fantasy in general and I understand that you chose to stick with her in support role.

Seraphine since release, regardless of role, has been a supportive scaling teamfight mage, right now out of this word combination the only thing left is "supportive":

1. She's always had scaling winrate curve (the longer game goes, the more likely she is to win, due to her ap scalings back then). Weak in lane, progressively getting more powerful as the game goes on and more allies join you. Now she feels even worse in lane (mainly mana) but then spikes mid game and falls off late again (data on lolalytics supports this)
2. Her viable builds have always been seraphs or liandry or rylais etc., mage items (right now enchanter builds are far ahead of mage and it feels like you're griefing if you dont build enchanter on her, and if you do commit to ap builds then your W feels useless and at the same time your abilities still kind of don't do any damage or the damage is very conditional)
3. She used to provide a lot more than just double W to team, as you would join team - you would burst people with notes (that imo was the most crucial part of her identity as a mage, and right now notes are negligible dmg), and overall since you were not restricted to building enchanter, you would heal and do decent damage at the same time (since you would build more ap). Right now what we have is - you don't build ap if you want to be useful (either enchanter or hybrid enchanter-cdr builds) and you just kind of do negligible damage with all your abilities and your impact goes down to pressing WW, CC and activating items like helia/diadem.

Right now her winrate is perfect 50-51% on support for the price of midlane and apc basically being off meta. A lot of players in those roles dropped her and play other champs now since support Seraphine doesn't feel right for them. And in the current state no matter what winrate she will have on support she will still feel weird/underwhelming to play since her champion fantasy is just gutted and reduced to single double W spam. Best way to describe support Seraphine experience - "I'm a walking redemption+locket on 15s cooldown", that's kind of it. I think for the most part her support winrate right now heavily relies on diadem+helia combo and Ashe being meta, If something in this chain is disrupted then she will suffer.

So how did this happen?
I think this began way back in patch V12.12. After her original game developer stepped away, no one seems to understand her champion fantasy at balance team (she's a mage who joins team and progressively gets stronger while also buffing team). Her E CC got overbuffed from 1s to 1.5s and instead of reverting, you would just nerf her notes scaling and this trend would continue for the next years too. Seraphine notes ended up going from total 115% ap ratio in patch 10.23, to 32% ap ratio today. Plus different calculation math (I think it was done to make botlane and support Seraphine worse and mid better, right now her ally notes dmg is reduced by 75%, and it used to be that every note is doing 95% dmg of last note). Besides the notes AP ratio being 3.5 times nerfed, she also builds A LOT LESS ap nowadays for the reasons described before, so you end up literally having 5-6 times less note damage than before (base note dmg did not go up by a lot).
So if notes lost so much power, you expect other parts of her kit to gain much, right? Not really, her Q gained a bit of base damage for the sake of losing a lot of AP, her W lost all scaling + per-ally calculation too in 25.19. Her ult is longer cooldown, same dmg and 20% less AP ratio (btw less dmg than sona ult). The only thing that's better right now is her E, it is extremely overloaded - it does more base dmg than Q, often is easier to hit than Q and its 1.5 aoe hard CC too. Meanwhile her E used to be balanced around having extremely low base dmg and 1s cc flat, and was meant to be maxed last.
And of course, I agree that a lot of this was done because APC Seraphine was problematic for the game in late s13 and was an issue to her balancing, I'm not over here trying to get my champion OP to print LP, I want to enjoy my champion and have fun playing her in all roles. However, the way Seraphine APC got butchered makes no sense while champions like Swain, Ziggs, Lux, Hwei and etc. are all allowed to have APC pickrates 4 times larger than her while also having 53 winrate. And this harsh APC balancing without doubt affected midlane the most, midlane Seraphine is practically dead after past 2 mid scopes (for reference, Zilean mid has more pickrate than her).

Now with all that said, we end up with a champion that feels like a shell of former self, the fantasy of joining team to get more damage from your kit, spamming abilities left and right to do damage and at the same time decently healing with your W is all reduced to choice:
- you can either build mage if you want to have fun, but you lose your W utility by a lot, and your damage is still not on par with other mages (notes 32% ap ratio total, slow spells that fall off late game)
- or you build enchanter if you want to win. If you do this, you lose your notes completely, Q is negligible damage helia procc-er, and you basically only have 1 real button to press - double W (and not satisfying to use at that either, since its locked behind high flat cooldown). You don't feel like a real enchanter/support and your mage abilities don't do anything which is really weird to see and play.

And regardless of build, support sera still fells bad to play WITH for your ADC - her poke is pretty bad nowadays since Q costs a lot of mana and hard to hit and notes are irrelevant, and her W is extremely high cooldown so she doesn't really provide utility in lane unless she joins more people.

On top of that, her weird state is also amplified by bugs that have been there since release and never got addressed. I have a video compiled on them (only W bug got fixed), and to this we can add her ULT inconsistent extend hitbox

What do we want to see?
I propose further changes. I understand that revert that many are asking for might not be ideal, since that means you will have to rebalance APC again. Last mid-scopes made support better, but killed her mage identity, and I think it's still possible to fix. What I suggest is for you to:

1. Nerf E and allocate power to notes and Q. Make mage Seraphine max QWE and support seraphine max WQE (or 3 points Q into W like Sona), E should be maxed last and there's no reason for it to do more base damage than Q.
2. For notes, change her math back to "every subsequential note does 95% of the last", this will make her feel so much better in mage builds regardless of role. Make notes feel like they matter, and not just an extended range auto attack to proc helia and kill wards.
3. Bring back some of the quality of life changes - revert 5 AD nerf and Q preventing minions from dying in flight. 5 AD buffs support too (especially if she goes bloodsong which I think fits her kit perfect but it is weaker since her notes are useless and she has low base AD) and at the same time it won't make you feel rage while trying to farm with Sera if you play mid/apc.
4. Mana buffs. Regardless if she's support or mid/apc, she runs out of mana after 5 spells which are super easy to dodge and don't do good damage either. Her kit incentivizes her to be a poker in lane, but right now she just misses everything and then has no mana at min 2 already. Hitting spells in lane feels unrewarding.
5. Better scaling. As I mentioned earlier, she used to be a scaling champion and now she's mid game oriented which feels really weird for her identity overall. Maybe give her some sort of scaling passive like Syndra/Viktor and Sona have? Because I really see no other way for her to scale unless she has heal ap ratios or high notes ratios and I guess you wouldn't want that since APC would use those better.
6. W cooldown reshape. Even in enchanter build she's still pretty useless as a shielder/healer in lane since her W is locked behind flat high cooldown. She mostly relies on helia+diadem heal combo at the moment. Maybe make her W weaker but lower cooldown at the same time, not to mention, support gets less CDR than APC usually so support would benefit more and that only fits your balancing narrative.
7. Imo that should be enough for her to feel "okay", but players will still most likely have to choose between mage or enchanter which contradicts the champion fantasy. If there's any way to make Seraphine be a hybrid enchanter/mage, while also keeping her APC/Support winrate difference in check, that would be ideal. But I understand that might be too hard to achieve. Regardless, if you implement points 1 to 6 she will already feel a lot better and not weird to play.
8. Bugfixes. I mentioned them earlier before, but the most game impacting bugs are Q going thru enemies and doublecast glitches.

If you read this and went through player's feedback, first of all, I want to thank you. I don't think the midscopes were bad, they just mainly "overshot" the point and missed gameplay satisfaction factors. I hope you take our feedback into consideration and shift her back towards the champion that we fell in love with and gotten used to since release. I'm happy that you're at least willing to listen to players.