Time Machine back up to remote disk. Is this possible anymore?

Hi, I have an old 2014 Mac Mini that I use as a Plex server and not much else. It is configured to back up via Time Machine to an an attached USB drive. This works as expected. I also have Macbook Pro M2 and a Macbook Air M1, both of which I would like to be able to back up to the same USB drive (the one attached to the Mini) over wifi. I have tried configuring this as described here [https://support.apple.com/en-gb/guide/mac-help/mchl31533145/mac](https://support.apple.com/en-gb/guide/mac-help/mchl31533145/mac) but the shared folder never shows up as a Time Machine target on either of the Macbooks. Is there a trick to this that I am missing or is this just not an option anymore?