Welcome to the forum!
As a Windows user, I can't help with the flatpack question. But I did want to mention this, a problem with using Latex, after it's installed (a bug):
https://forum.inkscapecommunity.com/index.php?topic=1163.0. So you might want to try installing TexText (or other similar, if there are any others).
Although I would expect the extension to at least be showing in the menu, after you have the INX and PY files properly in the correct folder. Even if you don't have all the other dependencies installed, it should be showing in the menu. Note that the INX and PY files have to be directly in the extensions folder, and not in another folder in the extensions folder. Also if there are more than one PY file, they all need to be there.
Hopefully a Linux user will be along pretty soon