TOPIC:

For what I read around, the DDRC-24 should upsample (or downsample) everything to a fixed 48khz.

Then, I read, it should be better to fix the source output to 48khz.

99% of my material is at 44khz so should set my player (which is Roon), to a fixed 48khz.

Is that a correct operation?

I doubt you'd notice any difference between the two but ultimately all you can do is try both and see what you think.

Most people would just leave the upsampling to the miniDSP, which is all I've ever done. Any difference would come from one device being better at doing this than another I think.