You can't even think of a better Grookey final stage than this.

Headbanging, rock and roll drummer gorilla.

Grass/Rock, HA: Rock Head
Bulky Physical Attacker
STAB Wood Hammer and Head Smash, Belly Drum

Rock Type works for the music pun as well as a reasonable material for drums/drumsticks/hammers.
First Rock Head user to get a useable dual stab.

Shame Gamefreak is too garbage to do this, but beats anything you anons have suggested.