@cheesekun:cheesekun.me I had investigated this in the past too! The answer is: I'm not certain. Previous to the post above where I show Gnome 3 working, Jay and I had largely only developed using the Mate Compton compositor, and that one doesn't send smaller crects than the entire screen. @jsorg71 had told me that most compositing window managers don't really send anything smaller than the entire screen anymore, and the bandwidth savings instead come from how efficient the differencing engine of the H264 protocol is.
All this is to say: The not-the-full-screen version of GFX has never been tested. Note that for RFX, we compute the regions ourselves, I don't think the window managers are sending us nicely-split-up sections, either: https://github.com/Nexarian/xorgxrdp/blob/mainline_merge/module/rdpCapture.c#L1141
Anyway yes, help is appreciated, and if we get this working my guess is this will be EVEN faster than the screaming performance I showcased above.
At the same time, I think it's worth investigating. Because we don't have the source to the Microsoft clients, your next best bet is to investigate if FreeRDP handles this type of thing (because maybe Windows servers do split up the crects), and then use that to see what can be done to import that logic into XRDP.