for me the blast radius is already big enough.
no.
not sure if he is immune to backstab.
what i would improve is:
-that he is doing more damage so he can insta kill the low hp classes (scout,sniper,medic,engineer)
-does a small explosion everytime he hits a player (like 30 dmg)
-make his rage animation a bit faster. sometimes the players can escape the blast radius before hes exploding