I also got some suggestions from the main Telegram group, here is a summary:
(in the case of a broken Ubuntu Touch installation)
- you can run the installer again
- untick the 'wipe userdata' box
- you don't need to reinstall any other ROM
This is ideal for my use case as I will actually be setting up software mainly on Libertine, Waydroid and Pocket VMs, which are containerised anyway, while the 'host' OS is for day-to-day essentials such as calling and reading documents.
I am sure taking snapshots/backups of containers is a lot more straightforward than the entire OS (which can be reinstalled to a guaranteed working state anyway): I haven't tried it yet, but with similar tools (Waydroid on Raspberry Pi OS and VirtualBox on MacOS), it was simply a means of copying the files somewhere else and moving them back when needed, so I'm sure it won't be any harder.
I will write again here if I find a way of performing incremental snapshots (a great storage space saver), or for any other useful information on this topic 