I've been occasionally playing around with Piano Booster for a good few years, but with the inclusion of the lessons by default in Ubuntu I've actually started getting serious about learning the piano recently. :) I have some comments about the song packs offered on the website but will leave my comments here related to this preview release.
I'm running the Linux version on Kubuntu 20.04 with KDE as the desktop environment.
The defaults you've set for fluidsynth are nice! This offers lower latency than my regular (48Khz) fluidsynth setup, and 22Khz isn't too noticeably worse in quality. :)
I have a 4K monitor and KDE is set to scale everything up to 200%. Unfortunately, Piano Booster does not seem to be able to handle this particularly well, as the fonts are properly shown in an increased size but everything else isn't. The effect is that the bars are very small and the time signature overlaps with both the accidentals and itself.

(Edit: had to resize the image a little, sorry.)
Another issue: when I'm in the Midi Setup window and press my keys, they do not play. This means I cannot preview the changes I'm making very easily. Also, when I close the window, all keys I had pressed while this modal was open will all play at once.
Finally, a bit of a possible usability improvement: in the Piano Keyboard settings, instead of entering a number, it would be nice if I can set the lowest and highest note by simply hitting the lowest and highest notes on the keyboard.
Keep up the great work! :)