I've filed a bug report with Eset, though those reports - that take the form of support tickets - are not externally accessible.
OK, so with the help of Deleaker, I've easily identified the main culprit: Beyond Compare. After blocking its two extensions (normal + UWP), everything seems to be quiet and peaceful — no more spikes of handles. I’ll keep an eye on it over the next few days.
What's strange is that, considering the popularity of BC among DOpus users, this issue doesn’t seem to be widespread 
Maybe there's something more to it?
I can add the following.
Discussion with people at Eset revealed the following.
- The Windows default file manager has a memory leak.
- Opus has a memory leak.
- Eset worsens both leaks (i.e., both 1 and 2) considerably. Yet, I am told that a forthcoming release of Eset products - perhaps the release-after-next - is going to fix 3, i.e., will stop Eset exacerbating the problem.
I do not know, or at least cannot remember, whether 2 is the case because of 1.
Did they give any details about what they think the leaks are in Opus (or Explorer)?
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I am afraid not. (That's closed source for you. And I had to push quite hard before my claim was taken seriously - though admittedly that happens in open source too. But, with your help, I provided reproducible cases with both Explorer and Opus. Still, given the layer of insulation that the base level of technical support provides to the real engineers, lord knows just what - if anything - has been done at the Eset side.)