Can you downgrade a 2021 iMac to Mojave?
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