I feel you :

https://www.pcgamer.com/valheim-multiplayer-dedicated-server/

This will do it for privately owned servers

You can do an anonymous login for running servers separate from your regular user account. Using the steamcmd tool you can install it with something like "steamcmd +login anonymous +force_install_dir yourinstalldirectoryhere +app_update 896660 +quit" then run it. I think it includes a little pdf manual for running the server. I run it on Linux so can't help much more than that though. Read here for steamcmd info https://developer.valvesoftware.com/wiki/SteamCMD

Edit: I guess I never tried running it on the same machine while playing the regular game so I don't know how that would work out.