Yh I've pulled out the rootball from the fabric pot and give it a smell and it doesn't smell foul or rancid just like dirt Yh they're never sat in run off I water till run off leave it five minutes then tip the run off in a bucket ....If it’s root rot get on yer hands n knees pull her out the pot a bit n give it a whiff. Owt that’s rotten has a honk to it. I’ll be honest, I’ve only been growing in earnest since 2020 though, I’ve only ever seen root rot from the media less hydro folks.
Your pots are raised and not sat in their run off or owt are they? If your sure it’s root problems and there no microherd to worry about then h2o2 I believe can be used