I would simply force everyone within a 120 feet range to join turn based movement. This prevents exploits while still giving multiplayer situations enough wiggle room if they split the party. 120 ft can also be crossed in just 2 turns of dashing. I absolutely see no reason to maintain partial turn based combat with others moving around combatants freely.
+1
You're outside of the range when combat starts and you have to spend your first turn or two just jogging? Well, next time don't split up the party that much.
