No real need for UST tyres with Flow rims. They’re designed to work with non-tubeless & tubeless ready tyres rather than being a full UST design. UST tyres are heavier and these days with good sealant you can run many regular tyres tubeless. Maxxis tyres seem to work particularly well tubeless even if not tubeless ready or UST.
As for tyres, HR2 as you say is draggy, but it grips well. I use it at the rear. Climbs just fine on roots and rocks. Drags on flat, smooth climbs and tarmac though, but as you say it provides confidence with the grip. HR does roll better, but a little more sketchy in the loose I find. Up front I prefer a Minion.
Ardent rolls well, but less grip on loose, crud and mud. It’s like a less knobbly HR really. I use it on my hard tail which gets a little more XC-ish riding or rather a little less of the weekend warrior style stuff. Lets me keep up with some of the speedy guys 😀
Ignitor is best left for groomed smooth fast dry trails in my opinion.
No idea about Panaracer. Never used them.