I'm no developer, just a general user, but if I had to compare 16.04 versus
18.04 - I had a lot of problems and stability issues in 18.04. For me 16.04
just works flawlessly, so I really enjoy using trisquel 8 (for more than a
year now everyday) with only minor problems and I'm actually not looking
forward to trisquel 9, which will be built on 18.04.
I actually used lubuntu 16.04 before with no problems, then upgraded to
18.04, had a ton of problems on my older devices, then I decided to try
trisquel 8 and became a happy person again.
Waiting for 20.04 to roll out to test it, but as 19.04 dropped support for
i386 platform, I'm afraid they also dropped support for older devices in
general, which I'm not happy about, because I don't like all the new hardware
spying technologies (AMT for example). There are a lot of pre-2010 computers
that run fine for casual everyday browsing and email. But however soon will
come a day, when there will be no up-to-date OS to run on them, which will be
a new security problem.
I apologize for an off-topic. Wishing you best luck in your effort to
develop Trisquel 9, truly libre and functional OS.