I think you should also do something about the proximity mode of the nades because its to simple to spam the flagroom with those proxynades.
Some solutions i thought might fit:
1.Decrease the amount of nades someone can carry
2.Make them destructible by weapon 8other nades rocket launchers etc.)
3.Increase the time they take until they explode after being ignited (which mean an enemy "activates the proxy mine and then for example only after 1,8 secs it goes off)