Deborah, maybe we are talking past each other. So easy to do when trying to share ideas in short replies.
As I understand, you have the Focusrite audio interface. With that you can plug an electric guitar in directly and record in an app like Reaper, GarageBand, or OBS. This you have done and shared recordings here. So all is good, other than maybe for a guitar lead that may have some intermittent failures.
So for the purpose of sharing to get feedback and encouragement you are good to go.
When it comes to the app to provide backing, I understand you can play songs in the app on your Mac Mini and hear it. Either the Mini has speakers or you have external speakers or headphones plugged into the Mini.
If that is all correct then my thinking is to figure out how to route the audio signal from the app that is being heard via the speakers into the recording software. Effectively the apps audio out becomes an audio source for the recording software.
Now I am a bit stuck as I don’t know Apple. That said, on my Windows PC I am able to set up OBS so that the audio output from an App is included as an audio input for recording purposes. I assume you could do the same on the Mac Mini in OBS.
You would also be able to setup OBS to receive audio input from the Focusrite. You would not have to spend extra time learning to synchronise anything. Just setup OBS to receive the two audio sources, start recording in OBS, start the app playing, and then play along. Only tricky thing can be if there is a really short time from starting the song in the app and starting to play along. I don’t know how long the count-in is on the app, if it is enough to go from clicking play in the app to being ready to play along.
I see you don’t have a phone. Given that it may be a simpler solution to buy a suitable USB webcam that can plug into the mini. That would have a built-in mic.
If I have understood all correctly, I think making use of OBS may be the simplest solution for making the recordings you want to make with backing.
But I may have misunderstood significantly.