I do fully agree that we need larger maps (imo every single toptier map should be 500m grid at minimum) however, making SPAA invulnerable to basically everything is not the correct way to go.
SPAA does infact threaten tanks, as it makes it harder for enemy team to spawn planes, and in turn easier for your team to spawn planes, which results in tanks getting bombed.
a less jank suggestion would be to just plain extend the play area behind the spawns. for manouvering safely out of spawn, for defending spawn, and for setting up your AA
Hell maybe stick 1-2 spawn points there for you to use if you feel the forward spawns have been pushed too hard by the enemy, kind of like an analogy to the forward and rearward airbases in air battles.