When you use the Flatpak version, you also need to get plugins via Flatpak, or work your way around the Flatpak sandbox (a flakpak application cannot access system-wide installed 3rd party software).
For the same reason Flatpak’ed Ardour cannot use JACK, either, but that is not usually an issue.
Yes, Ardour supports VST2 and since version 6.5 also VST3 on all platforms, and u-he plugins are know to work.
Still you can get a demo of Ardour and demo versions of u-he, so best try before you buy,
which is always a good idea.