combine whatever treatment with being in a cool dry environment,
Everything points to sweat, moisture.
The days not working out I’m outside, grass to cut, dogs gotta get out and about EVERY day, grandkids ...
Only cotton boxers if any and shorts most of the year where possible. Air flow seems to help.
Hire a kid to do yardwork.
You have to rearrange your life around never sweating to avoid reversing any progress you have made.
An idea I came up with in school is to put on clean socks first when changing clothes, THEN boxers. Eliminates feet to crotch transmission of any nasties... even if there are no apparent foot problems.