I think the action econ/big bad problems could be settled fairly simply, and not detract with Legendary Actions (or make them ubiquitous. Let the legendary things keep their legendary stuff and be legendary).
PCs get: Action, Movement, Possibly 1 Bonus Action (from class feature, feat, what have you). And possibly 1 Reaction per round (also by class features, feat, whatever).
Foes/NPCs/Monsters -yes, ALL/ANY of them: Action. Movement. Bonus Action... and can make 1 Reaction per round up to a maximum equal to each HD they possess (minimum 1).
4 against 1 guy with 5HD? They can take a Reaction up to 5 times per round...Since there are only 4 guys attacking him, this only allows for 4. But if the party was 6 PCs, the BBEG could "hit back" (or run away, cast a quickie spell, whatever is allowed for "Reactions") 5 times.
Also, just for me, I'd wish the Bard was made a half-caster and had a spelllist that was significantly more focused and included more Nature/druidy stuff.