This is replacing a bad system, with another system that is equally bad, if not worse.

These long capture times when you build anything, coupled with time and cost of respawn (10sup, so even stock base warrants 50 respawns), makes it impossible to cap bases.
So strategy to hold the base atm is:

1. Take key points,
2. Build as much as you can on them
3. Have 2-3 players (defenders respawn) and try to do at least 1 for 1 trade, there is plenty of time to interrupt the timer
4. By the time your spawn protection wears off, you already get enough supplies back to respawn again.
5. This will stop any attack

Strategy to take such place:

1. Bring at least half of your team
2. Radio backpack
3. Have 1 or 2 ppl disassemble everything while others protect them
4. Everyone out of the cap zone
5. Everyone back in
6. You will cap at slightly higher pace
7. Pray to sweet lord Jesus that more defenders don't join in
8. Take it, if you haven't lost most of your team by then

If ya wanna keep the longer timer to reward supplying and building, i'd suggest increasing respawn time on base, if its contested (enemy within base building perimeter), from 10 to 30 seconds.
This is still 2x faster than radio backpack but it will reduce respawn frequency. Also increasing the cost of spawn.
At this time its impossible to drain base of supplies, to prevent enemies from constantly spawning.

A simple can't spawn when there are x players within x meters of the spawn point is the way to go. it is simple, it has no downside, it makes sense.

That means unless you have people constantly manning bases (there is alot of them), you will loose them.
And you only have theoretical 64 ppl at best, while defending AI is hardly a challange.
This will only create alot of base back and forth
Take lose, take lose, take, defend, lose another one

Those attacking the base should not show up until they have captured it.
Otherwise, even one defender can stop the attack

Most likely, speeding up the capture with a large number of attackers will improve the situation
Or increasing the spawn time after each death

Another way of handling it could be that if the number of attackers outmatch the defenders inside the base, respawning is disabled, and on the other hand if the defenders outmatch the attackers, respawn is still allowed. This encourages grouping up into attack teams to overwhelm a base and prevent the defenders from respawning. Any defenders inside of an overwhelmed base would need to be very careful, as respawning isn't possible at that time. If enough attackers/defenders are killed, their advantage/disadvantage changes.