I figure a 'solution' is to have big boss fights not be everyone doing their biggest attack, but having more of a maneuver and support element.
If it's a big dragon then a PC using their greatsword to propel another PC to smack it in the head, a PC grappling one of it's arms to make an opening for the other PC to stab at the chest. A PC stabs the dragon in the foot to prevent it from trampling everyone so the other PC's can get a shot at its weakballs. One guy is buffing or debuffing, the other then strikes.
If it's 6 dudes vs one evil dude in a big flat room then it shouldn't be easy to just have all the ranged guys shoot and the melee guys surround evil dude. So the one guy is able to maneuver in a way that keeps a PC in the line of fire.
The action economy imbalance seems like it can be solved with having more AoE's baked into the system. Everyone can do a spinny melee attack at penalty, everyone bigger can trample anyone smaller as a move action. Wizardry Tale of the Forsaken Land has casters being able to 'at-will' channel arcane energy into a partner's weapon, so stronger enemies take up two character's worth of actions for an attack with a good chance of harming them.
So the basic idea is the PC's can fight well as individuals, but against boss threats they must voltron their actions together to bypass high defenses, nullify "it will kill you in one hit" actions, and so on. Figuring out what the "deathblow" of a boss monster is and what actions are needed to reduce it's potency can also be part of the info gathering phase leading to the fight.
![Image](https://lparchive.org/Chrono-Trigger-%28by-Mega64%29/Update%2018/97-113.png)