TIL: VESA Standard does not necessarily mean that VESA components actually fit.
Ah well, I was going to handle this one like a raw egg at all times anyway.
Had to mcGyver the shit out of this backplate to make it somewhat fit to the new monitor back. Found some decently sized spacers. Screws were a little bit too long so I had to saw ~5mm off but now the monitor arm plate sits snug and flush to the display.
Let's see how long this holds up. Hope there is no bad pressure point somewhere.