@c4pp4 Thank you for the reply!
The anbox-ubuntu-touch package was available all of a sudden a week or so ago. Got everything installed and running.
Perhaps this was all related to the OTA updates.

First off: big thanks for the efforts in porting Ubuntu Touch towards the Xperia X - gave the device a brand new life for me! (after unused Sailfish OS installation)

Secondly: I seem to be confused about the possibility to have Anbox on the Xperia X. Seems I do not have any repository that contains the "anbox-ubuntu-touch" binaries, no is there the "anbox-tool" binary preinstalled on the image.
If I read @c4pp4 's reply right, however, anbox-ubuntu-touch should be able to install via apt.
I have the ubports repo added via /etc/apt/source.list.d and apparantly am on the devel update channel fredldotme and receive updates, latest 2020-04-28/2.

Thanks in advance for any pointers!