Alright I'm back, and I successfully got a build down after spending a couple more evenings on this.
Robert, 2 things.
1) I grabbed the zips from github on the last couple versions, please be aware it looks like xaudio2.h from \3rdparty\compat\mingw is missing (at least when getting them zip style)
2) I've been using MAME64UI for what feels like over a decade, is there a place I can donate to help support it? It has brought me a lot of joy over the years.
And... if some fellow newbie comes across this and would like to know what I eventually did that actually worked, here you go, gather round so I can regale my conveniently numbered steps for ye:
1) go to https://www.mamedev.org/tools/ and grab msys64 bundle and unzip it into a folder
2) rename or delete the folder mingw64
3) grab GCC 10.1 from Robert's site: https://messui.1emulation.com/
4) take the mingw64 folder from that package and place it where the msys64 one used to be
5) inside your msys64 folder, run autorebase.bat
6) inside your msys64 folder, run win32env.bat, which gives you a command prompt with the needed paths/bindings/what have you
7) in that command prompt, go to the folder where you unzipped MAMEUI64 source code
(I tried version 245 and the latest, since the latest sounded iffy on how MAME is changing, but both worked for me, maybe stick to 245 if things go haywire)
8 ) type this and press enter: make OSD=messui PTR64=1
9) If you get a compiler error that says xaudio2.h is missing, grab it from an older source code collection for MAMEUI or a newer one for MAME itself (for me they were all version 1.6 when I looked in the file), find it in \3rdparty\compat\mingw and copy it to the same location as the code you are attempting to compile
And that's it, good luck out there! Off to go re-write some popmessage()-s for me.