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A note on repositories
For some reason I have noticed a tendency for Ubuntu users to add 3rd party repositories to their installations. This is not how apt works, and should never be done. This 'feature' has fallen into abuse for the most part. To get later versions of packaged you either use snaps (or some equivalent) or you use backports in apt. Adding a 3rd party repository can degrade security, stability or both. It also causes your Linux install to fall out of the straight and narrow path, much like the dwarves who left the path in Mirkwood where their fate was almost sealed by such folly, we must be ever vigilant against doing such things. Please review “the software and updates” package and the “other software” section and ensure you don't have any 3rd party repositories. If you do, review why you have them, remove the software you installed from them and then remove them, and instead get the software via snap if you need it.
There is one exception, the ppa graphics driver team for Ubuntu provide newer graphics drivers that you might need to support your Graphics hardware. This repository can be left if required.
Upgrading from stable to the next testing release
This is very easy. Both Ubuntu and Debian have a single file to modify, assuming you have not added 3rd party repositories, and a single command can then bring your install to the testing version. On Ubuntu this is Ubuntu+1 and on Debian this is Debian testing.
As you have already configured Timeshift, this is safe since you can just roll back if anything breaks. See the Timeshift section on how to do this (and configure if you have not yet don that).
