Update on my upgrade plans (as in: downsizing): Been nearly giving up, because the dimensions reported do not match up with reality - yes, there is 65 mm "space" on the GPU side, but its bought using the same hack as with the Meshroom S: slightly "bulged out" side panels. The bottom panel itself only got about 140ish mm of working surface, because there is a recessed area to both sides which hold the holes for the hooks of the slide-in panels (those reduce the inner surface size by about 6 - 7 mm). On the GPU side this means, there is only about 50 mm space both on top and bottom to be used for attaching anything, so if the radiator isnt ultra-small, and the GPU is basically single-slot or 1 1/2 slot, there is no chance to eg. hide the GPU behind the fans of the radiator etc.
So I decided putting the case aside for a while.
.. and came right back to it the other day, because I remembered my "raise the top" idea. After a bit of fiddling around with a Silent Wings 3 140 (a leftover from the installation of the Silent Loop 2 in my Meshlicious Streaming System), and a Scythe Kaze Flex 120, plus some other small helpers, I figured out that there is, at best, 28ish mm space from the mb / gpu tray to the top of the case. An extremely small radiator could fit here, eg. the Silverstona Vida 240. And then somehow add fans on top of the case.
The other option would be: Cut open the space where we want the AIO radiator + fans be placed, remove parts of the holding frame which allows for the handle to be attached (and OC remove the handle in the process as well), mount the fans to the pre-existing fan brackets, and put the radiator right on top, with custom drilled holes or just happy friend ziptie to the rescue (or BOTH). Finally, create an additional top with similar color scheme, ie. cream-white, either using steel / aluminium, or 3d printing, with a nice mesh lid, and attach it over the radiator.
Optionally lower the position of the mb by up to 5 mm, and then remove the proud corner of the MB tray, that'd normally also keep the inlet for the mainboard spacer, and drill holes for the spacer attachments at the lowered positions. That'd also mean removing a bit of the IO panel cutout, thats why I'm saying "up to 5 mm", because it might be limited to just 2 - 3 mm of material removal.
I'd be going with an all-white AIO, probably NZXT or Alpenföhn Gletscherwasser, 240 mm radiator, and just reuse my P12 Max or the SW 4 HS that I've got sitting around doing mostly nothing (being used as case fans).
In this, this is also my VERY FIRST WHITE case, and white-themed build! 
cu, w0lf.
ps: on term of the original topic, ie. Mesh-mATX, I'm considering moving the streaming system to my Meshroom S, and reuse the soon to be vacant MSI Z690M, get another 32 GB DDR5, et voila! And then of corpse use the B760i to build a much smaller NAS, together with the components of the SAMA-IM NAS system, eg. using the Jonsbo N2. .. or sell a few parts, who knows?
