I've tried a variety of things: kneeling chair (these work well but create different kinds of discomfort for me); donut cushion (works, but also creates its own kind of discomfort). Presently, I am trying a simple extra thick memory foam cushion (15x16x4") and I'm liking it pretty well. I.e., just make the chair you're sitting on "more cushiony."

Standing desk converter is still my go-to, but as you say, standing all day can be its own difficulty.

Side question. All I got was a prostate exam. Can this confirm infection?

It cannot. In first world countries, it's largely obsolete. You should have some basic imaging done (ultrasound at a minimum) for preventative care reasons. But this can't determine an infection, either. You should do a urine and/or semen culture.

Which abx were you prescribed?