Is there an objectively superior firmware to be running on my M30S+'s in an immersion tank?
You can contact their support on telegram
https://t.me/whatsminersupport and request the liquid-cooled firmware, you will also need the AirToLiquid tool (Whatminer tools alternative) for immersion-cooled miners, and ya, as Artemis3 mentioned; this will void the miner's warranty as the firmware lacks many protection mechanisms which originally exist on the air cooled firmware.
Same question for the OS
OS for what exactly? your question is not clear.