The UI works on the Pro¹, yes. I believe the distribution I show in the video was entirely installed form the UI.

The installation of the X session is based on scripts, and theoretically could be made working on all distributions, but for now they only work on Debian or Debian based distributions (I personally tested Debian Sid and Kali, both work fine), more work would be needed to make them work on other distributions. Attach (terminal) works for all distributions I think.

If you want to use another distribution with XWayland, just have a look at the scripts here, which are also available in /mnt/guest of the installed distribution. Distributions which are recognized as based on Debian will use setups/debian.sh, but you could make other scripts for other distribution types, and maybe run them manually if they are not automatically called I suppose. I haven't investigated but is not necessarily much work, probably mainly systemd and package lines to change to comply with the package manager and services the target distribution is using, possibly some different dependency names too. Illyria didn't start working on other distributions yet because he first wants to improve the base script, and then try to extend it to other distros.

Did you use the "init container config" and "setup xsession" buttons at the bottom of the container view in harbour-containers, advanced controls? Theoretically they should build Xwayland which should take about 10 minutes, then setup DE/WM, and after that you should be able to start the Xsession from the button above.

For i3 (which I think is a lot more convenient to use on the Pro¹ since it reduces the need to tap accurately on the screen for basic actions), there are other things to do but they are more specific to i3 in general, not LXC, like ~/.Xresources, ~/.xinitrc, and a good base configuration. I plan on contributing to the harbour-containers' wiki to put the instructions together but haven't found the motivation yet, in the mean time I think any i3 wiki would do and then perhaps some tries and errors to adjust font size and such.