Probably JSON. I haven’t been involved in Flatpak for a long time but I’ve never seen XML. JSON is quite close to XML in it’s layout sometimes I find so easily mistaken.
The driver installation has got a lot easier over time, still shit that you have to install a driver, still shit support for older cards. The open drivers they're building are too little too late for me. They didn't care about my slightly older GPU so I stopped buying their hardware. All AMD/Intel from here on in.
Generally using only a few flatpaks is where it's generally "bloaty". Adding more actually balances out the equation ans you have more apps using generally shared runtimes.
Probably JSON. I haven’t been involved in Flatpak for a long time but I’ve never seen XML. JSON is quite close to XML in it’s layout sometimes I find so easily mistaken.