Thanks a lot for your answers.
TL;DR - the most time is spent dealing with projects that themselves are under resourced and no longer participating with the same enthusiasm they once were! This applies to the whole simrel + epp.
For the 2020-12 release cycle, the biggest single use of my time (by far!) was trying to fix the JAXB issue that caused problems when different sets of plug-ins were installed. The conclusion of this seems to be that Mylyn (not wikitext) is just going to be dropped from simrel, and therefore all the packages. It took me days (5+) to resolve and test this issue and continued right through RC2 as the final (known) problem was discovered in RC1.
For that, I would blame SimRel, its "kindness" and false promises. So far, EPP used to assume that SimRel bits could be taken together and work; this time, it has been clear that it's not the case: SimRel is not really a reliable source of consistently working bits. The fact that SimRel doesn't actively prune projects that lacks enthusiasm leads to SimRel becoming a kind of pot-pourri of projects with many of them in a non-industrial grade intrinsic or support quality.
The fact that EPP got such bug, and it was detected only in EPP months later and not by SimRel itself before it released is a bad smell.
I don't think any technology can fix that in general. It's an organizational issue to bring to Planning Council. As you are probably aware, I personally advocate for the progressive drops of SimRel as it doesn't appear profitable these days, and as I believe dropping SimRel would allow to focus more energy in EPP, where I believe there is more quality checks and better ROI. This story makes my case against SimRel thicker ;)
However, in parallel of the general analysis, I'm curious: is the dependency issue a case that's likely to happen again?