Coud probably be done. Find publicly shared Immich albums -> sync to Google Photos -> share publicly -> write the public Google Photos URL to the metadata of the Immich album.

Another could be an insulated public facing Immich instance that gets publicly shared albums pushed to it, the same way as with the other scheme. No reverse access to the private Immich instance. Then if that gets hacked, only those photos leak, attacker can't get their hands on other server data. Assuming it's really well isolated. A public VPS would probably be best for this.