My RG35XX+ has been sitting in a box since I got it, more or less, because there was no adequate custom firmware to replace the stock. I like MinUI but it has no wifi functionality, which is most of the "plus" in RG35XX plus.
I don't get on with Batocera / Kokiri for several reasons, but the start-up time is the deal-breaker. I can't be waiting 20 seconds for a thing to boot, then remembering to take it out of sleep mode and charge it every day.
I've seen some videos recently about muOS which is not as nice as MinUI in terms of its frontend, but is lightweight and fast to boot, which makes it acceptable for my purposes. Getting the network services running properly was a bit of a rollercoaster, but a staic IP and some random combination of reboots and SD card swapping finally put it right.
So here I am, tweaking things in retroarch and poking around in theme creators and the muOS discord server wondering if I should start learning C.
There's a filebrowser (dropbear?) included in muOS, which is convenient for up/downloading files at <deviceIP>:9090/. There's also sync software called syncthing, which I haven't managed to get working yet, but which could be nice, especially since I'm hopping between devices quite a lot at the moment.