IMO, Assassin is a pretty worthless class, especially so when Rogues exist. Lethality is so incredibly unreliable that it may as well not exist, and having the thief ability is more useful 100% of the time. It would take some pretty significant stat difference for them to be considered worth using, so I personally wouldn’t worry too much about raising the Str cap higher.
Swordmasters and heroes are stronger than the pony and knight combo. Assassins are “masters of the killing arts” so should do at least as good, not worse than pony knights who aren’t combat masters. At least that’s my reasoning
I definitely feel that strength cap of 20 is too low for a sword locked unit like the FE7/FE8 assassins. 23 or 24 seem to be reasonable caps; though, it would depend on the caps of other units. I would say at least on par with falcon knight strength cap.
I am more or less fine with a low defense cap. It makes sense for the class. Maybe 22 Def and 20 Res?
My perspective on this is that, if you want characters to be more tanky options of a particular class, you’d solve that by raising their defensive growths. Raising the class caps doesn’t really fix that issue, and if you’re going at this from the point of “every unit should have maxed out stats” then a few points between caps don’t really mean as much.
I’ve never been fond of how weak the assassin’s caps are either, so I typically raise them a little, but I also balance out the characters who can be that class’ growths too, providing diversity in the type of assassin you want. For example, Jaffar is all strength and skill, Matthew would be speed and luck, and Legault would be the tanky one. That said, thematically, I’ll be honest, assassins shouldn’t really be tanking much of anything. Not physically anyway. I mean, just look at their design; no armor. Even peg/falco knights have armor.
What you should be considering here is whether or not you have characters that will even naturally come close to the caps you want. Most assassins by default do not, in terms of defense and resistance. It’s the archetype. Much like how knights and generals will cap their defense stats a lot, but rarely pass 12-15 speed, yet they still have 24+ for their speed caps. The number being that high is relatively pointless in most cases because most generals won’t even come close to reaching it.
If you want my numbers, I’d say strength 22, defense 21, resistance 21-23 depending on whether or not you have to have an assassin be a mage-fighting class. Which if that’s the case, then their strength doesn’t really need to be much higher considering magic class units tend to have lower defense regardless.
(You could also remove the 15+ Crit bonus on Swordmasters, change the bonus to 10+ and add it to Assassins, that would make them unquestionable very deadly I think, and make the speed 3 or 4 lower then a Swordmaster.)