If you want a proper server, it seems that Asrock Rack is the only manufacturer of AM4-socket-based server mainboards. Unlike desktop/gamer boards, these are designed for parallel airflow, typically from front to back in a 19" rack. These also come with IPMI remote maintenance, so can be operated headless in a remote location.
I have considered one of these for a while, such as the X570D4U, which also supports up to 128 GB of ECC RAM. Depending on what you want, this may be overkill, though.
(This was my favourite, because it has two M.2 slots, but there are others with only a single slot, since you said you only need one.)
Unlike gamer or other boards, these have no fancy black vanity covers and often won't allow overclocking, but are typically very well designed and rock solid for unattended 24/7 operation.