A 1-3 second timer for /back would be the perfect median between donators and non-donators. This way, /back is still very effective while not being to be abused for combat or raids. The very command itself is powerful enough, so a 2 second cooldown wouldn’t remove its great effect. It would simply make it hard to abuse with by it not being instant.

EDIT: Or put a return cooldown on, this way /back is still instant, but a player can’t spam it to go back and forth through bases, picking up their dropped items before 2 seconds pass. As Sara mentioned, /back isn’t just used for pvp, but also to escape dangers such as drowning or lava. So putting a return cooldown wouldn’t hinder the effects of escaping non-pvp dangers, while still removing the possibility of pvpers to abuse the command.