Sorry about not addressing your query about such groups earlier because I usually avoid signing to such groups/events as timing has been a bigger impediment for me for tackling backlog, so I didn't think that joining timed events would help much. Moreover, some groups announce their monthly rules publicly, so if I feel like it, I can always pursue such events silently. I just joined backlog assassins to track my progress on my own. But you should definitely try and see if these group events help you.
Also, on top of all the other issues mentioned earler, I play a lot of simulation games like Cities Skylines and Transport Fever 2, which like Rimworld, are games not to be finished, but you can fire up a new map any time and sink in dozens of hours... Currently I am trying to find games from my library that I want to play and can possibly finish within a few days/weeks... To that end, Wildermyth turned out quite grindy, but I'm still working on it.... and recently finished Rime and most of TF2, where I lost campaign progress because I didn't realize it didn't have cloud save...