Chess clock / egg timer. If they don’t decide fast enough then no action for them! You don’t even need to actually do this (it would be a bit extreme in most situations), but the threat that it might happen should give them the necessary kick. Too much time deliberating? This monster just remembered it can take a legendary action on your turn!
Or my experience with D&D…
Encounter giant pit monster
Everyone argues for 20 mins about how everyone else should use their turn
Repeat until monster is dealt with
Run out of time of the the evening, so go home
Chess clock / egg timer. If they don’t decide fast enough then no action for them! You don’t even need to actually do this (it would be a bit extreme in most situations), but the threat that it might happen should give them the necessary kick. Too much time deliberating? This monster just remembered it can take a legendary action on your turn!