I'm not a Mac guy so Brian can correct me without worrying about hurting my feelings.
A. The Mac Mini's preferences don't need to choose the Moon USB DSD HD Audio. You just need to set MC to use it in "Options > Audio > Audio Device". MC will only use the system preferences when it is set to play to the default audio device. Otherwise, it plays to the Audio Device it is set to play to, ignoring the System Preferences setting. By all means, set the System Preferences to the Moon USB DSD HD Audio/SimAudio 240i integrated amp when MC isn't running, so it is used at that time.
EDIT: I am assuming that the SimAudio 240i integrated amp presents as Moon USB DSD HD Audio in the Mac preferences, and also in MC. If not, please clarify. But what you want to do is select the SimAudio 240i integrated amp as the output device in MC.
B. Use Internal Volume.
C. Have MC maximize System Volume when playing from MC, so that it doesn't affect Internal Volume's control. "Options > Audio > Audio Device > Device Settings > Maximize device volume during playback".
D. I set the Startup Volume to 80% and the Maximum Volume to 100% in "Options > Audio > Volume". I also set the Internal volume reference level to 80%, which is just my preference.
E. With those settings in place, use the physical volume control on the SimAudio 240i integrated amp to set a comfortable listening level, and thereafter use MC's Internal Volume control.
I would recommend leaving Audio in Exclusive Mode, and just don't run MC at the same time as Apple Music, Safari or Pure Vinyl. Without MC running you will be able to play to the SimAudio 240i integrated amp.
Again, corrections welcome.