I'd call Marvin not just a "good" but a
great ebook app, though still far from perfect.
I can see now that the TTS feature is listed in Marvin's public bug tracker / feature request collector:
https://github.com/kguil/Marvin-Roadmap/issues/84
So, one can only hope.
As to "only EPUB", that is exactly how it should be. I convert each and every e-book I have into EPUB -- the only proper e-book format for this day and age, in my opinion. All the other e-book formats are either obsolete or bloated (looking at you, Amazon).