Several of us are rolling fat true 20" rims–mine’s a 47 mm wide KH with a 406 mm bead diameter, not the 387 mm trials size aka 19", and the CST Operative is a nice tire at a nice price for that. Tip of the hat to jtrops who clued me in to it. My uni originally had an Onza Sticky Fingers tire on it, which has knobs and is almost as wide as a true trials tire. It was more fun for hopping than the Operative, but the Operative is more useful for all-around geezer freestyle and flatland type practice. And to be truthful, I’m hopping better now on the Operative than I ever did on the Onza.
20" wheels are mostly just pretty miserable on rough surfaces. But I’d think that a tire with lots of volume would help as much as anything.
You could do as well to get a trials frame and build up a 20" wheel for it like the one kahunacohen did in this thread, especially if you already had some take-off parts on hand like a saddle or cranks from another unicycle that you could apply to it. Or start with a used trials uni, or maybe one of those Koxx’s on Amazon for just over $200. (Haven’t heard much about them since they first showed up there.)