My project runs on Quest 2 at 18 FPS (GPU-bound), no matter the scene or XR Rig

After days of troubleshooting, I can imagine you are ready to be done with the hunt for the culprit. 

I am glad you are making headway and found some good results. I would like to mention that 60 FPS on an empty scene might not leave you much room to be able to develop your app. Ideally you should see 72 FPS on an empty scene. 

I uploaded a couple of screenshots to my google drive, in case you would like to compare the settings now or in the future if it comes up again. Cesar's Google Drive - Unity6.2 Settings.

With these settings (active hand tracking + Interactable in the scene) I am getting a steady 72 fps in the standalone build.

I did notice a few differences between your setup and mine, though it could just be due to Unity version changes. From what I could see in your screenshots, one of your custom package renderers seems set as the default URP pipeline (In Graphics). I’d try switching back to the standard one, just in case it’s overriding performance settings. It’s also worth deleting higher-quality tiers you’re not using, so the project can’t default to them accidentally.

I hope these help, and if you need anything else let me know.