That does sound like a good approach. My only question and it might be a tough one to answer, if it makes it worse should I put back in the old fluid and filter? It's not running that badly now, I don't quite get it. There is a chance that new fluid might flush out valuable debris, metal filings, whatever and it will slip even with the old fluid put back in.
In a lousy dilemma, the cleanest boneyard SDL I've ever seen (3 weeks ago) is still on the Portland South Pick n' Pull yard. I was passing through, not much time, had to pick up riders going south. I got a couple of things, oh my God so much more would be worth a looksee. No idea if the tranny is still on it. They don't give you that info I don't think. I'm going to have to be ready to pop a tranny pan any time I find one.
https://www.picknpull.com/check-inventory/vehicle-details/WDBCB25D0GA252137