That's all good to hear! Sounds like you know what you're doing
Sorry if these all seem like obvious questions, we just have to cover all the basics since we don't know how experienced someone is when they make a post, and so often fin rot turns out to be a water quality or cycling issue, I'm sure you know.
@Colin_T knows a lot more than I do, but personally, I would use the salt water treatment in his main tank (and I wouldn't worry about the plants, mine have certainly survived salt treatment before) and do daily water changes, re-dosing the salt, for two weeks, and see if he improves.
If he doesn't respond to salt treatment, I believe persistent fin rot needs antibiotics to heal. We're restricted in what meds we can get in the UK, but eSHa-2000 works against fin rot, fungus and bacterial infections. Using some right now on some cories and seems to be helping, and eSHa worming meds also worked when I needed to treat my livebearers.
He's a pretty boy!