But I think my absolute favourite way to do classes is how Cypher handles it. You get your broad archetype and your narrow specialty, and the two are completely independent of each other. Sure, some combinations might be less optimal, but there's nothing stopping you from being a bard who burns people's sins.
However that's really hard to do, and probably not very OSR. So I suggest sticking with kits or class groups/subclasses. Or going for a feat system of you want to have even more customisation, but you're again getting away from OSR.