Hot take: I gave up on sphagnum moss for my pings after 3 of them rotted in one week
I used to swear by long fiber sphagnum for all my Mexican butterworts until three of them turned to mush back in April. A grower at the San Jose cactus show told me to try a gritty mix of pumice, sand, and just a pinch of peat instead, and I grudgingly repotted my whole tray. Two of the sad ones came back and the third is pushing new leaves now, so I guess I was wrong about the moss. What mix are you all using for pings that sit on a windowsill?
What was the humidity like on that windowsill when the moss ones turned? I only ask because I've had the total opposite thing happen, my moss tray dries out way faster than my gritty tray so rot has never been an issue for me, but my place stays pretty dry in winter. The sandy pumice mix sounds solid for wick watering though, do you bottom water or soak from the top with that combo? I've been wanting to switch a few of mine over but I'm scared they'll dry out too fast under a south window.