Hello, I'm the guy that wrote the Realm667 tank you mentioned in the original post (The sprites weren't made by me, they were made by Gorman Freebmane).
Like Enjay asked, what are you actually looking for, tank assets or code ? Based on what you said so far it sounds like code is what you are interested in. In which case, there really isn't much out there*, there are implementations like Brutal Doom's tanks or BOA's and the likes, but those definitely fall under being too complicated, even the code for the old tank on R667 is fairly complex, despite how basic** it is, so like Enjay said, any implementation of an actor with a turret that is mostly independent of the actor carrying it will basically be complex and difficult to do by default.
* Nor are there that many tank sprites out there, most of them are edits of the Riot Tank from Duke Nukem 3D, including the ones from Iron Snail. The tanks that Freebmane made for my mod are basically the only original ones I have ever even found so far (For now

)
** And janky, terse, and broken for that matter, I'll be fixing that at some point.