If you are able to get ethernet to your Mac Mini, then do that.
For your speakers, you’ll need to find a wireless endpoint that can stream high-def audio and connect it directly to your speakers . There are many different solutions from a 2nd router or node, mesh device, network audio player, rasberry Pi device, etc.
Here is an old post about endpoints. There may be some better options available now, but this should give you some ideas of what’s available.