Recommendations

• Valve Index but it’s not standalone. It’s “just” a headset so what you do with it is up to you, no need to register anything with Valve, nor have Steam (even though SteamVR is convenient) and works on Linux. With Proton even the latest indie VR games work. IMHO also Half-life: Alyx is in itself worth it.
• Lynx-XR1 that can already be rooted lynx.miraheze.org/wiki/Rooting_Process (which I did few months ago) but arguably their customization of Android isn’t as convenient as what Meta did. For tinkerers though and if you can actually get one, it’s great. Also no account required.

There are others, e.g. simulavr.com which is standalone and Linux proper (not Android), but I haven’t tried so I can’t vouch for them.