Not familiar with PF's Three-Action System (i only dabbled with PF and it seemed no different from 3E to me).That's what I meant. Can you agree with any of this? Because, IMO, these are all problems that make combat less fun for my table, but could be fixed if WotC in 5.5e/or a backwards-compatible 6e did something like Pathfinder 2e's Three-Action System or something else to fix these issues.
I guess I can't agree that its a problem to worry abut too much? I don't know. Some of those things might be mitigated by house rules that we implemented that had more to do with what made sense to us rather than thinking about an "economy." For example, I allow bonus actions to be completed as actions as long as you don't break the "no two non-cantrip spells in a round rule" because I see no reason why if something can be done as a bonus action it can't be done as an action.
Or like the solo monster issue, if it is just any ole monster sometimes the party stomping them before they get to act twice is fine and fun and feels very accomplished! Or, it is an important monster I give it some legendary actions (I am a very strong believer in every edition - that each monster is potentially unique - and if they seem identical that is because of my shorthand not because they "actually" are in an in-story way. But generally, in my experience, even solo monsters potentially benefit from the environment and the goals/stakes of the fight.