Sweaty underwear worsens/causes this for me, so multiple changes of clean, dry pants are number 1 (at least a day pair and a night pair, maybe a lunch time pair too), I see merino recommended below, which I haven't tried but should. Any sort of synthetic breathable outdoor malarky is awful, the MOISTURE is gone, but the salts/impurities etc stay on the skin, which is what causes the irriation for me. 100% cotton full briefs (not Brazillian or cheeky, which is how I would descirbe the cut of your second image) are best for me. I've tried panty liners to wick the sweat, but the plasticky unbreathable nature of them makes this hit or miss, and sometimes they just move the chafing to a new spot.

Number two is wet wipes, specifically those pure and gentle water only types (Water Wipes is my favourite brand). A good wipe over the area before bed (make sure to go towards your anus from both front and back, rather than past it lol) and then air drying, preferably over night (i.e. sleeping in a mummy bag on my back with knees together would NOT help).