Hello, I too am using UBT 16.04 on my Nexus 7 flo. Although its eol, its still great for web browsing and minor development. In my case, it was easy installing UBT on my Nexus 7 but harder installing UBT 20.04 on my Pixel 3a. Installation definetely varies as the Nexus 7 stops at Android 7ish and my pixel came with Android 12. I had to downgrade to Android 9 to make it compatible with UBT and then I had to flash UBT like 3 times before it was a successful install.
FWIW IMHO the Nexus 7 2013 models are good devices BUT with the lack of maintainers it makes it really hard to hang out on, considering the device support % score; you may want to move to a device that is not only UBT compatible but one that doesn't ship with anything past Android 9 because the downgrade process is tricky and risky if not careful.