Figured it out. Or at least got it working again. First of all I got SteamVR working again by deleting a lot of cache files by following this guide:
Then when I got SteamVR working ok, I tried OpenXR, and this time it loaded and I could use the OpenXR toolkit menu! And the fix for the 5fps problem was enabling “turbo-mode” in the OpenXR toolkit menu. Instant spike to 120fps in the menu, and my in-game fps was back to normal! 