Yep, it should work fine with a read-only photo volume as it doesn't make any changes to photo files by default - it just needs to write to the DB which is configured as another container in the Docker Compose file.
Unfortunately I ran into issues with the docker-compose.yml method of running Photonix. I'll give it another try later and create a github issue for you. Thank you for replying to my question!
You might just want to try making a copy of a smaller selection of your photo library in a new folder and giving that to Photonix to try out. That way it doesn't matter if anything were to get removed.