Can you downgrade a 2021 iMac to Mojave? We work on an older SAN that will not run on OS beyond Mojave.  And yes, if money was no option and I could replace 400TB with a more modern system I absolutely would.  However, it is what it is and I would like to get the best system I can for our current environment.
Any advice would be much appreciated.
You can‘t run old macOS versions on newer hardware, there usually are some drivers missing. There are special constellations in some cases (and we probably have some answers beneath similar questions) but as of now it‘s highly unlikely that there is a way for a 2020/21 iMac to run Mojave.
Options which may work for you

Run Mojave in a VM and access the SAN from there
Get an older Mac mini/iMac capable of running Mojave via eBay or similar