If I may add onto the boss fight plan, we could try a combination of the suggested ideas, so that everyone has minor control over the boss like starting simple attacks or responding to their player character's own attacks. The GM/chosen player will have full control to do more significant actions, for example, actions that completely change the surrounding environment, or anything that brings the boss fight to a new "level" (a sudden powerful self-buff, or a wide area of effect magic spell, for example).

Example of what a regular player can do: Launch attack against the boss, boss is hit by the attack, possibly retaliates (focus is on the player's character).

Example of what the GM/chosen player does (in addition to the above): Destroys surrounding buildings and towers with an AOE attack, forcing player characters to take evasive actions in the next round of posts (focus on own character, and significant events that affect all players).


This way, each player has the flexibility to launch attacks with their own characters and have the boss respond within a single post, so waiting for the boss player isn't necessary. Having a GM/player with full control over the boss will allow more significant events to happen to keep players on their toes, such as changing the difficulty or pace of the battle if needed.

All in all, this should be a good way to keep things simple and moving smoothly.