Definitely go with a community OS build of your choice. More information on why at the links below…
Questions 2, 3, and 4 have relevant answers scattered around the forums from posts over the past few years. I’d recommend searching a bit. Battery life and performance have definitely been discussed a lot, from the best sorts of batteries to use, to scripts and configuration settings to adjust for better performance or battery life. It’s also worth noting that for the Rex builds especially, some of the Devterm information may be in the uConsole section of the forum since the OS builds for both the uConsole and the Devterm are the same in that particular build (but with minor settings adjustments to activate one or the other, or CM4 vs CM5).
Question 3 is rather vague, but lots of posts have been made over the years with either software people have successfully used on the Devterm, or hardware mods, or 3D printed case mods, etc. Some modifcations have been quite elaborate, like the cartridge based hardware system someone made to support the printer and other add-ons like ethernet/serial ports and other stuff.
Regarding java, anything that can be run on a Raspberry Pi 4 (assuming you have the CM4 version and not the A06 version) should work. I haven’t used java recently, but I’m pretty sure using the Oracle version requires a lot of extra setup since it’s not in the usual debian repositories. The version that is the default and is in the repositories may not be 100% compatible with the Oracle version, so that could be an issue depending on what you need to use. Maybe that’s not as much of an issue now, as my experience with it in a Pi was from maybe 5+ years ago.
While these are focused on uConsole, the suggestions mostly apply to the Devterm as well:
Definitely worth diving into those links and considering some of those things! 