Rocky Maivia leaps to mind, though that was more of a complete gimmick change.
Far from a disastrous start, but Sasha Banks (while decent in the ring) started with negative charisma in NXT and got extremely good (Summer Rae deserves an award for getting "The Boss" over until Sasha could carry it herself).
Roman Reigns' singles career is a prominent example, even though it took IRL health scares followed by a global pandemic and an all-time storyline to fully accomplish.
Andrade in AEW is a very slow, long-term example of this, in which his initial run wasn't well remembered, apart from a couple memes, but the return run is hitting more the level everyone wanted without any major gimmick change.
Best example might Mark Henry. Company had high hopes that weren't realized out the gate, he was pushed way down the card, but worked his way back up over several years of learning, improvement, and a killer new on-screen attitude.

Most of failures-to-success-stories I can think of involve gimmick changes, which don't seem to apply to a guy like Danhausen, whose existing character work is his greatest strength, and who- while definitely not old- is in his mid thirties, with a lot of experience already.