Fennec is not maintained by Divested, but who they actually are maintained by is a mystery. I hear a mix of Mozilla themselves and that it's just firefox with different build scripts. The F-droid page links to everything firefox except the issue tracker, which links to here.
IMO it doesn't go far enough with deblobbing and removing nonfree software, but for the time being i'll switch to it. Hope someone continues this project.
Most UIs are not optimized for touch, but if you use one that is like GNOME it will still have issues. The onscreen keyboard is not outstanding, you essentially need wayland apps for modern features like touch scrolling etc, and there will be bugs. For me the main bugs are some GTK popups will cause them to crash when typing into them on virtual keyboard, and swiping through apps doesn't work very well on debian. If you use a different de or distro, your experience might vary.
Tablet linux is not perfect yet, I'd suggest a pixel tablet w/ custom roms unless you are willing to use a slightly buggy linux interface. If you are willing to, look at dell latitude tablets or surfaces
The hardest part about the setup you're describing is the power. PCs aren't as power efficient as phones, so you'll need to lug a battery around with you if you want it running at all times.
I want to warn you before you consider touchscreens: Linux is 90% of the way there. I own a tablet, and it works fine, SIM is detected, but there are just those little bugs that will drive you insane.
I didn't either.