If it works for you without issues, then I don't see a good reason not to keep using it.
Personally, when the software that I want is not available on the repositories, or is too out of date for my needs, I try to find it elsewhere. Typically the authors of the software would have additional options such as a PPA (basically a private repository) or a direct download to the .deb that I can install manually. Be very careful with installing software like this.
Only then I install as Flatpak, and if I find that I want to keep using it regularly I would try to compile from the source code.
As for keeping packages pinned to a particular version, I've never tried but it seems pretty straight forward based on this thread:
Which is quite similar to how you'd do it for regular .deb packages: