So I played around with this a bit again (My Zenfone2’s battery is finally only limping along, making it infeasible to use when not plugged into a power source for more than 5 minutes).
Termux works great, but is a non standard Linux environment, thus you can’t easily get a lot of stuff working. But together with AnLinux (use the ad-less version from Fdroid) you can easily install Ubuntu18.04LTS in a Proot container through Termux. Seems to be working as expected.
Next up, I’ll try to get external SSH access and dhcp client working through USB tethering (by default Android acts as a dhcp server) in order to use it as a normal Linux server in my network.
This seems to give some hint for dhcp client network:
Edit: hmm, seems like Termux also supports installing Ubuntu20.04 directly without AnLinux:
https://wiki.termux.com/wiki/PRoot
edit2: no i686 image for Ubuntu20.04 though it seems.
Edit: for reverse tethering, root is required.