Yep, agree with Sabato - Trump is a de facto leader of the opposition right now so his approvals will affect the GOpP’s performance. We are in unprecedented territory. There’s no way out for the GOP either beyond Trump just shutting up, which he won’t do.
If - and I'm still saying IF - this drags the GOP down, nobody of significance in the party will dare to call him out for it. And the orange buffoon himself completely lacks any that he's a gigantic albatross for the GOP.
Imagine any other defeated prez meddling in the subsequent midterms... really shows how unique his post-presidency really is.