As well as a time limit have a distance from spawn limit - maybe 10m?
so spawn protection ceases when EITHER the time or distance limit is reached
If you ensure spawn points cannot be made close to various other points of interest - esp cap points - then that would address some of the problems apparently caused by the time limit alone.
True. Monastery Invasion map, for instance: if you are playing the attackers, the last point (the monastery itself) will cost you most of your reinforcements if the defenders are playing cheeky and build a spawn inside a house directly opposite the gates.
You are basically in the open, they are inside the house and effectively invulnerable.
If they manage to do that before you get any teams inside the monastery, you will have a very hard time getting past them (unless they are complete trash, ofc.)