Well, grenades are probably quite realistic. It takes five i think to disable the tank. In real life you probably could break the tread of a tank with five grenades and thereby immobilizing it. The problem is when you compare with pfaust, arty, and airstrike. 1 pfaust should IRL easily immobilize a tank, a well placed shot should destroy it totally. For arty/airstrike direct hits would destroy it, near misses at least disable it.
Since destroying it totally is not in the game, making the panzerfaust and perhaps arty a little bit better at immobilizing the tank would be a good balance.
Or, as you indicate, making it some 15 nades to immobilize it is perhaps more in line with how much damage you can take in the game vs IRL. For example 3 headshots kills you in the game 1 headshot IRL. 6 bodyshots in game, perhaps 1-2 bodyshots kills you IRL.