I already discussed it on Discord with a few people. We have complained enough about PL already so I’ll skip that part. However what limits every alliance further is the inability to pre-invade while at PL. You take away a vital tool for an alliance to support “weaker” members.
I think it would be good to allow pre-invasions when at planet limit, with a penalty for all I care. Since it would be a failed invasion by default you are bound to loose your invasion ship(s). Although I doubt that will count as a penalty later on in the game.