I think the concern is overstated, Embiid was the MVP runner-up and the Bucks' bigs feasted from penetration, penetration, and some more penetration.
I believe Gallo provides plenty of post offense but if the feeling is that you find turnaround jump hooks more pleasing than jumpers then might I suggest Enes Kanter? Did you shudder? Of course you did.
JaVale McGee fits the bill the best from a money/minutes standpoint. His early career was marred by being the total opposite of mental maturity but he's been a cog on two championship teams recently, particularly the familiar Warriors. Maybe he can offer something in the locker room plus be able to fill in as an emergency C?
You may shudder again but... if you're looking for a C option that can be deployed a lil more often than McGee due to age, for matchups, foul trouble, even spot start here and there due to injuries/rest then our old buddy Alex Len could help.
Overall, the 4 man rotation of John/Gallo and Cap/Gwu is beyond solid. Gwu seems set to be Cap's eventual replacement so a 3rd vet C would be sufficient for now and then you can stick Knight in the GLeague as the 4th C. I'd draft a PF to be Gallo's eventual replacement or even John's. I have a strong belief John is brought back even if it might be with the idea of trading him down the line in the not so distant future.
As for the rest of the off-season, I don't see Dunn opting out but maybe a healthy summer and contract year motivation gets him back to the player he was. Because of the player he was, I don't see another non-shooting guard being added (sorry T.J.). I'd personally like a bigger scoring combo guard like Austin Rivers then Schlenk can play musical chairs with Snell or any of the plethora of wing shooters.