I have a 2018 Mini that still runs fine on Mojave, and will proceed "no further" than that (I still use lots of 32 bit software).
The first thing you'll want to do is BACK UP anything important on the Mini as it is now.
I'd suggest CarbonCopyCloner, which is FREE to download and use for 30 days.
It will create a backup that is "finder-mountable" -- just connect and use it as you would use any other external drive.
There's a special key combination you can use with internet recovery:
shift option command R
This is supposed to install the original OS that shipped with the particular Mac you have. The 2018 originally shipped with Mojave. So, I'd try that.
If you're connected via wifi, you'll also need your wifi password.
(ethernet -- no password necessary)
It will take a little while for the internet utilities to load.
When you get to the finder, open disk utility.
IMPORTANT: go to the view menu and choose "show all devices".
Look at the list on the left.
The "topmost" line is the physical drive inside the Mini.
Click on it to select it, then choose "erase".
Erase to "APFS with GUID partition format".
When the erase is done, quit disk utility and open the OS installer.
Begin "clicking through".
Be aware that the Mini may reboot one or more times.
The screen may go dark for several minutes.
BE PATIENT.
When the install is done, you should see the initial setup screen ("choose your language"). Begin "clicking through" the setup process.
When setup assistant asks if you wish to migrate data from another volume, you can TRY this:
Connect the backup and see if setup assistant will recognize it and offer you the option to migrate some data from it.
BE AWARE:
Setup assistant MAY NOT WORK because you're migrating from a volume that has a "newer version" of the OS than the drive to which you're attempting to migrate.
If setup assistant won't cooperate, there are "other ways".
We can discuss them in another post.
For now, just finish creating the account with a username, password, etc.
