What if you only got one action per turn, but a round wasn't complete until all actions had been exhausted?
For instance, you have the ability to perform an Action, a Bonus Action, and move. So initiative is rolled once, but you cycle through and only choose to do one thing. So you use 10 ft. of movement and end your turn and it cycles to the next person's initiative. They take an Action and end their turn. Monsters move up to 20 ft. and end their turn, and it's back to the top of the initiative where the person who moved 10 ft. uses their Action to attack the, now in range, monsters. Then the person who used their Action, moves their entire movement, 30 ft. and since they have no bonus actions, their round is up unless they get a reaction, but the PC who only used 10 ft. of movement still has 20 ft. of movement left and the monsters have 10 ft. and their actions.
It'd mostly be difficult for the DM since they'd have to keep track of multiple creature's round breakdowns, but definitely doable.
For instance, you have the ability to perform an Action, a Bonus Action, and move. So initiative is rolled once, but you cycle through and only choose to do one thing. So you use 10 ft. of movement and end your turn and it cycles to the next person's initiative. They take an Action and end their turn. Monsters move up to 20 ft. and end their turn, and it's back to the top of the initiative where the person who moved 10 ft. uses their Action to attack the, now in range, monsters. Then the person who used their Action, moves their entire movement, 30 ft. and since they have no bonus actions, their round is up unless they get a reaction, but the PC who only used 10 ft. of movement still has 20 ft. of movement left and the monsters have 10 ft. and their actions.
It'd mostly be difficult for the DM since they'd have to keep track of multiple creature's round breakdowns, but definitely doable.