Thanks for sharing the pictures of your WIP.
I’m also interested by the my64 mini-itx solution, but in a second time. I planned to use an old PC chassis where the mini-ITX will have plenty of space and I will see what comes up. I also planned to use an old Asus Radeon HD6450 Silent (1GB DDR3) because it has no fan, just a dissipator on top of the GPU and it’s a low profile card (only powered through PCI bus, below the maximum 75 W). It will be of course less powerful than your RX550, but I think it’s sufficient for the desktop stuff I plan to do.
About your GPU card’s choice I was wondering: the air exhaust will tap into the keyboard’s PCB right ? Then there might be some thermal dissipation issues, depending on the load put on the GPU. It will throttle in that case I guess. And the typing experience might be unpleasant if it’s getting too hot below the keyboard.
In all cases I’ll follow your experience here as I might buy this case as well.