The older HL radios that are not the 6.0 suck. Bad. Yes after market controllers work much better. Clark boards are quite popular but are a bit whacky. I use and older IBU2 which isnt made any more. I dont like the new IBU2 Ulitmate system they make now.
Im told by my friend fast tanks who uses lots of HL sytems the HL 6.0 is a really nice system and it supports a servo recoil.

You can remove the airoft gun from the HL sherman and get a 3d printed servo recoil unit from Clark and take the barrel and put it into it and have servo recoil and the stock elevation motor or get the one that also has a servo for elevation as well. Hope that helps.
Go direct if you want more help, [email protected]
Regards
RichJohnson