Now I know this sounds like "one weird tip" and "vets hate me"😭 but I just have to make this public service announcement.
So I've recently been struggling with three or four cats (I'm with a rescue) that all had chronic diarrhoea. We did all the tests - all negative. Did Panacur, a course of metronidazole just in case, on gastrointestinal food, added probiotics etc etc. basically the works. Some things improved matters slightly, but it was only marginal.
Then someone said they'd had success by giving hairball food. I thought "well, nothing to lose". After four days on Royal Canin Hairball dry (apparently Purina hairball also works) and they were all producing the most perfect poo. Six weeks later and there has been no relapse. I'm experimenting now by introducing small amounts of other food and that is trial and error, but some other food is now tolerated.
You're welcome 😁