Get a used HTC Vive. The Pro variant even holds up reasonably well with modern headsets due to the higher resolution and the OLED screens. Both work with Monado.
Virtual Reality
Virtual Reality - Quest, PCVR, PSVR2, Pico, Mixed Reality, ect. Open discussion of all VR platforms, games, and apps.
This is more off-topic, but what exactly is Monado? On their website they talk about all kinds of crazy stuff, like implementing vision-pro-like spatial interfaces. That makes it sound like its an OS, but its not, as far as I can tell.
It's an open source alternative to SteamVR. Basically a set of drivers and configuration utilities.
Woah, that seems really cool! Do we have something like an overlay yet?
Overlay AFAIK not, but there is this GUI app: https://gitlab.com/gabmus/envision
Right! I tried installing (building) this one before, but Debian 12 didn't have the right version of gtk4 so it wouldn't build. But thank you for sharing, I'll give it another try!
I strongly recommend checking out the Linux VR Adventures Wiki.
For an overlay, it's probaly the easiest to start with wlx-overlay-s .
Google cardboard?