At no point did the OP suggest that Rainmeter doesn't work for him with the hardware acceleration turned on. He only said that it made the flickering worse, that's all - just stating the facts. My impression is that this is a driver related issue. The flickering, not just of Rainmeter, but of any app using the GPU a bit more intensively, is common to driver-less or driver-wrong operating system, and I've met it a lot of times in my debugging operations over the years.
That being said, I have a question for you, given that the topic is somewhat related to this and that you probably have more experience with this than I currently do: I'm trying to implement UsageMonitor's in my skins (well, the possibility to choose displaying this alias was always there, I just didn't think of testing it until now, given the Windows 10 Fall Creators Update or later requirement), and only get with around 1% to 2% usage, nothing else. The screenshot you posted here appears to indicate that there should be more processes being displayed in the "Top GPU Meter", so my question is: am I doing something wrong to get only appearing to use my GPU, or it's just a tricky PerfMon counter that might provide inconsistent numbers or processes here, depending on the large variety of GPU cards?
By the way, I did read in your skins that should be excluded from this, but I'm sure the fact that I'm not (yet) excluding it is not the cause for this, since, after all, the is not even shown, and displaying only as using the GPU happens in your "gadget" as well.
Maybe you know some things that I don't regarding this, and could shed a light into this odd behavior, maybe?