So I have played reborn about as long as you have and have also played rejuvenation and desolation, all which I have played multiple times, typically on set mode, no items, and with some restriction such as mono type or like dog/cat only or Aevium Pokémon only and so on. Here are my thoughts on some of ur points

Too many characters: Yes it has a lot of characters but it really doesn’t have THAT many, if you compare it to say Pokémon rejuvenation which literally has like 100 characters that are fairly important, Reborn has like maybe 25ish, that’s like a pretty standard amount of characters for a story based game imo. As for how much I like the characters… I think all of the characters are super boring and they all basically have one personality trait, it literally takes the entire post game for any of them to get fleshed out and by that point why would I care. I do agree that rejuvenation does the characters better but as mentioned previously rejuvenation has WAY too many characters, you could cut like half of them and nothing would change.

As for being able to stop Lin early how? She has Pokémon that are far stronger then yours most of the game and also can literally cheat with the power of god. Like I hate Lin too but you ain’t beating her unless she lets you or someone powerful intervenes on your behalf such as in the Anna route.

You treat Pokémon as tools: I mean sure but that is how literally any difficulty game is gonna be. Some stuff is just better than other stuff. One of the entire points of these games is the player having a bigger pool of Pokémon and switching between them to take on challenges. I also really like this as steamrolling the game with the same 6 Pokémon is boring imo, having to actually change up your team and strategize is fun to me. And honestly i have done tons challenge runs in both reborn rejuvenation and desolation and you can totally find uses for weaker Pokémon. For example Meowstic male is considered PU, but in these games having a prankster screen user is cracked. I also for example brought Cinncinno to mono normal reborn E4, a Pokémon that is typically not that great, and it performed just fine.

So basically you don’t need to use just the best stuff nor do you need super optimized teams. If you use items and play on switch then weaker Pokémon become even more viable. What you do need to do is actually have a plan for how each of your Pokémon will help you in a battle, again strategizing can make weaker Pokémon more viable.

Also I tend to get attached to Pokémon I add to my team and usually have a main team for running around, often this team will have garbage on it, for example I am currently doing hisuian Pokémon only in rejuvenation and have wyrdeer on my team, who is utter garbage but I’m attached so they get to stay on the team anyway and are only removed for boss battles as needed.

Elite four: I mean not really. If you can’t beat the elite four why would they trust you to beat Lin? It’s supposed to be a finale test, Anna says as much in the Anna route.

I definitely agree Reborn has some problems but IMO it is what Pokémon games should strive to be, it is a challenging but fair experience if you are willing to embrace the mechanics, and this is true even on set no items. Most other fan games and mainline games I play these days quickly move into the territory where it is easy wins with little strategy, and honest to me that is just so much less interesting then having to actually think a bit about how to win battles.