Illness/Sickness: Recurring fungus despite good water parameters and temp

My axolotl Gummy has recently had a bit of a recurring problem with gill fungus, which is causing them to atrophy and is impacting his appetite (he's lost a lot of weight). This is especially distressing because his partner died a month or two ago.

The water temperature hasn't tipped over 19.5°C at the absolute maximum, so that's not the issue and the ammonia/nitrite/nitrate are at 0/0/5, so that seems fine too.

I've started doing salt baths (they're as heartbreaking for me as they are stressful for him) and daily water changes, but does anyone have any suggestions as to why this might be happening? He thrashes about so much when he gets put into the salt bath or back into the tank that I'm worried that it's going to be doing more harm than good. Even after having no sign of it for a couple of days, it was back today on a gill and all over one eye!

Sorry for the loss of your other axolotl
Is there anything in the tank that the fungus could be attaching to?

You could try putting an Indian almond leaf in his tank. The tannins tighten the skin so the fungus can’t take hold to him as easily.

Do you check your general hardness?

You can give tea baths for fungus too, it's a lot less stressful on them.

If you axolotl has white patches on it's body that's an indicator of early fungus (easier to tell on darker coloured axies) and for that I'd use a tea bath, save salt for the fluffy gill fungus

If the fungus is not getting in because of temp or water parameters it might just be that it appears to be gone but is reinfecting him as soon as it looks cleared. In this case, I'd agree with Hayleyy and suggest tea baths, as they'd tighten his skin and hopefully stop the fungus taking hold.
It might also be where you're storing your dechlorinated water - if you're not sure you can empty them and rinse them out with boiling hot water before refilling.