Yes, Jon, it appears you're right – Marvin once again seems to be dying (as I've just mentioned in the other recent thread).
And to me, the lack of standard Beta-testing for Marvin 3, and pretending as if the many dozens, perhaps hundreds, of unresolved issues posted to GitHub, didn't exist, were early warning signs. It's certainly nice we got the Marvin 3 upgrade, but it appears to have been a fortuitous, one-off event. The recent 3.1 version is a marginal improvement (I don't use GoodReads in conjunction with Marvin, so it brought nothing for me). At the same time, there are still deficiencies in Marvin 3 introduced during the transition from Marvin 2, that still aren't fixed, and it doesn't seem they ever will be.
What can we do? I'm still sticking with Marvin for now, because it still offers several unique features not available in competing e-readers, but I'm certainly also forced to look at alternatives now, such as Hyphen and MapleRead. For 5 years or so now, some of us have been waiting for the no. 1 crucial missing feature to be added to Marvin –
syncing annotations. (Really, in today's world, when most people own both a phone and a tablet, the lack of annotations syncing cannot be excused away.) Some have already ditched Marvin due to this missing feature (and they also require annotations syncing with their desktop environments), but I'm still holding on to Marvin for now, painfully collating, manually, my annotations from the various devices whenever I finish reading a book. But it's definitely not a long-term, professional solution.