sorry for the late reply. Yeah I've never grinded on lunatic+, I didn't want to buy the grinding dlc out of principle and I don't have the patience to do spotpass grinding on lunatic. I honestly doubt grinding would even help that much unless I did a massive detour, e.g. reclassing everyone to Dark Flier for Galeforce and back again. Units hit stat caps early and lategame maps can be tricky even with stat caps.
Really the problem with all the characters I listed is weapon ranks- namely, they don't start with the capacity to attack at 2 range. Stahl has Discipline, but he starts with E lances and almost D swords- he's miles away from being able to throw Javelins. The vast majority of training opportunities on lunatic+ rely on the unit being trained being able to attack at 2-range while a bulky unit like Robin holds a choke; for example, every Shepherd bar Stahl and Vaike can train in chapter 3, by having Robin block off the stairs while your trainee hurls weapons over Robin's head. In my last playthrough I was able to get Sully from base level to level 8 in chapter 3, and with a C Chrom pairup she was doubling every enemy in chapter 4 besides the boss.
Most units in awakening are like that- their growths are so good and their bases so bad, the turnaround when you feed them levels is massive. Units without 2-range are notably harder to train, and while they do generally have something special once they're trained- Donny and Panne have ludicrous growths even by awakening standards, Stahl has a flawless class set, Gaius is incredibly handsome- it's not really enough to make it worth your time.
Though honestly, anyone is usable when you're going at such a slow pace. I imagine you could feed Stahl a few pokes with the bronze lance in chapter 2 and chapter 3, then do all the old Donny Training Tricks in paralogue 1 to get him to D lances and a few levels. By then he should be strong enough to fit into the chapter 5 turtle formation in interceptor's guide, you can feed him the whole chapter and hey he's good now.
maybe I'll have to give him another try some day.