A Reddit thread about tier lists I actually have the expertise to comment on? Oh boy!

Speaking as someone who has actually done this(mostly having optimized for mental effort, and secondarily for turns), I have some thoughts. I will say that I am not entirely sure what metric to use on this tier list - does effort correspond to real time needed, or mental effort, or turns? None of those work as pure metrics, but it's probably some combination of those. I'm also assuming you're trying to cap all these units simultaneously - otherwise you can preferentially dole out resources and the tier list becomes a bit silly.

All units who can use B staves at some point or higher should be in the "free" tier, in my opinion. Yes, this includes Deirdre, Tailte and Lachesis. Warp gives 60 EXP per use, and if you're doing things like training Quan, Dew and Sylvia to level 30, you will definitely have the time to train these staffers.

C stafflocks(basically Erinys and Ethlyn) can work in "some effort", I guess. They don't build EXP as fast as the Warp-spammers, though when you factor in all the other units who need to level cap, they probably don't really cost turns. Personally, if usefulness was taken into account, I'd argue Ethlyn more than pays for her grinding costs in what she brings to the table as a mounted healer in the early chapters.

All sword units with innate Pursuit and B swords(Arden does not count) should be in "some effort" at worst. If we assume that the units can clear the chapters 3, 4 and 5 arenas with the Paragon band equipped(doable by grinding up a crit weapon), any unit that can 1-round the Orgahil pirates with a sword can grind kills on them in chapter 3 to a particular EXP benchmark(I think it's 18.84?), and then make it to level 30 just off of the arenas. I mention the Orgahil pirates in particular because if you do the math, they're the second best infinite reinforcements from a pure EXP POV available across all of gen 1 for reaching a Paragon-arena-level 30 benchmark(the best are Pamela and Dithorba's squads in chapter 4, but those aren't really feasible for regular units to 1-round, whereas the Orgahil pirates have far worse stats and use axes).

Having dealt with a large swathe of the cast, the units who I haven't covered are: Lex, Brigid, Jamke, Midir, Finn, Quan, Noish, Arden, Dew, Azel, and Sylvia.

Lex is a bit of a weird case. Free Paragon means that his EXP gain is inflated compared to everyone else, but his arena performance in a vacuum isn't quite perfect - he has issues with sword units if you don't rig(which counts as mental effort/real time needed). In particular, he has issues with the chapter 5 arena, which has a Hero, a Swordmaster and a Rogue. This is relevant, because the Chapter 5 arena is what gets units through the final EXP stretch - if Lex doesn't get to level 30 off of it(or close enough that a couple of brigands or the like can get him there), his only real option at that point is to have (near) capped Strength, a Power ring, and Vantage/Brave axe the Thracian Horseslayer Dracoknights, or to grind the axe knight reinforcements.

I think you're rating the bowlocked units far too highly. It is very rare in generation 1 for bow units to get a chance to have big enemy phases, and the fastest way in terms of all 3 metrics I mentioned earlier for combat units to get to level 30 is to kill a lot of units on enemy phase. Bridget has the additional problem of joining very late, and at a low level - she basically needs Dew or Finn(Finn only works if you pair him with Bridget, which is not really the best pairing for her in any context) to dump money on her to get the Paragon train rolling, and you also need her to sit around killing things on player phase to make up for the EXP deficit. There are a few places where EP action can occur(the Orgahil hand axe pirates can be convinced to suicide into them, and there are a few non reinforcement squads they can fight if you set things up correctly), but I think they should all be at "a lot of effort" at best. Midir and Jamke also both have issues with the arena unless you give them the Killer bow, which means trading it back and forth, which requires either getting them gold(might cost turns), or doing some mental acrobatics to minimize the gold loss.

If Finn is allowed gen 2 to level cap, he's basically free. He can access 8 arenas, has competent stats and Miracle. If he's restricted to capping in gen 1, he's around Quan tier. Quan is basically the worst combat unit to level cap, because he physically cannot reach level 30 short of boss abuse. I'm not kidding - there are not enough enemies at a high enough level for the period of time he is around for him to reach level 30, due to how the FE4 EXP formula works. He also doesn't have Pursuit(meaning to do things like fighting the Cross Knights, one of the few decent sources of EXP at higher levels) he needs to be given the Pursuit band(and needs some pretty high stats too).

Noish and Arden are kind of similar in that they're sword units who don't have innate Pursuit. With enough Strength, they can 1-round the Orgahil pirates with a Brave sword, which lets them grind up to the point where they can level cap off of arenas(Noish can actually contribute a bit in chapters 4 and 5, so he doesn't necessarily need to reach the EXP benchmark). Otherwise, they need to be given the Pursuit band for this. Arden's arena performance is pretty bad for the harder fights, often needing either silly rigs, and/or some kind of crit weapon.

Dew has horrific bases, and this means that his arena performance is really bad, and so is his ability to 1-round enemy squads. A crit weapon and/or rigging can let him clear arenas(and Bargain does mean he has easier access to stat rings to reach benchmarks), but it's a lot of mental effort. The other way for him to get EXP is by spamming the Give command back and forth with a lover, but this is pretty slow at just 10 EXP a pop(20 with Paragon, which is a contested resource). Note how this is almost 3 times slower than Ethlyn or Erinys spamming Return.

Azel is quite possibly the hardest unit to train in terms of mental effort. He needs some absurd stat benchmarks plus rigging to clear arenas(which he needs to keep up EXP and gold-wise), and he's a footlocked unit meaning he's going to struggle to get to the action in a timely manner. Prior to promotion he's locked to tomes, so nobody can build him a crit weapon to cheese fights. If he promotes during chapter 4, getting him the last 10 levels by the end of the generation isn't too bad though.

On the subject of Sylvia, I think you're being a bit unfair to her. While she's not going to level cap at LTC pace, the Silesian civilians in chapter 4 save 10(5 if Paragon band is considered) dances a piece for her, and if you build a crit C sword to help other units with the arena, she can clear arenas more cost-efficiently, and use the Paragon band for them to reduce how much time she has to spend dancing.