Two changes could avoid new players (or killed players) getting killed before they could do anything:
1) A counter on screen showing how many seconds you have until other players can hit you (10-15, I think are enough to spend all of your points and get going). This fully shielded period already exists, but you don't know when it ends and is now very short, observing other players.
2) Implementing one big rule: only players below level 15 can hit other players below level 15, so no one with an upgraded tank will be able to hit players with the basic model (tier zero).
3) I think that tanks under level 15 and tanks above level 30 should be immune to each other. And if someone is waiting for your immunity to go out, you can boost them out of your view range.