You have a ton of options.

Rounds last three seconds. If they aren't paying attention or act too slowly, bam, time is up. Always talk a player or two ahead: "Hey player A, you're first in initiative. Player B, you're next, get ready!"

Start playing with timed events. Build a small session or two based on puzzles and timing. Slap down an hourglass and go! You can always bring this back when people slack. 10 ton rolling boulders outright kill just about anything, especially when it's restrained from the boulder.

Offer to let PCs guest as NPCs so they don't come to every session, but show up once in a while and play an NPC within parameters you set. I had my very distracting roommate play the barkeep - he started to like the game more, cared about his bar/Inn, and got a lot more into positive RP elements of letting people have their time to shine, not interrupting, and leaning into weakness (I sure as shit didn't let a small town barkeep get above level 3, and I gave them a low point buy - it was an NPC, so they initially didn't give a shit about the character or its stats).

Or just boot them. Apologize, but try to share your difficulty in maintaining ALL of the work it is for you.