You might try posting your question in the Bicycle Mechanics subforum.

I would guess that your rear derailleur's behavior is a symptom, not a cause. Perhaps you have a malfunctioning freehub or freewheel. If you have the latter, given the limited supply of freewheels, it may be time to switch to a wheel with a freehub and cassette cogs. Unless you have a fabulously expensive rim--doubtful--freehub service probably doesn't make sense, in which case you could move the cassette (if it's not showing signs of wear) to a replacement wheel.

Place the bike in a workstand or elevate the rear wheel, then spin the wheel. The chain, cogs, and crank should not move. If they do, you would appear to be confronting a freehub/freewheel problem.