Thanks for passing on the suggestion.
I have uninstalled Groupy after trying it for a few weeks. I gave it a good go, but found it awkward to use and unpredictable in its behaviour. I thought at first that this was just my learning curve, but then noticed that when using saved sets, some tabs wouldn't always open (browser and application windows, for example), I knew it wasn't just me.
This flaky behaviour alone was enough to convince me not to waste any more time on Groupy, but the real disappointment was its inability to use and save complex search criteria in File Explorer (as detailed in my original post).
I'll continue to be a big fan of Start 10, but there's nothing else that Stardock has to tempt me with right now and my experience with Groupy has reinforced my opinion that Stardock is a one-trick pony.