Hello Tony; I would go with the SSD because those Apple SSDs these days are super fast and your programs and OS will be running off that drive so it will keep things nice and snappy. As for the 5TB USB3 Pro Tools Session Drive I would reconsider that and perhaps consider using that drive as an archive drive but put current and very recent sessions on another external SSD drive that is say 512GB to 1TB (or even straight on the Mac Mini) depending on budget. Then when certain sessions haven’t been touched in a while you can move them over to the archive disk. If you don’t want to do the archive drive then consider doing 1TB+ in SSD as your Pro Tools Session drive and punt the archive storage issue for a while down the road when you really need it. This drive doesn’t have to be blazing fast but you don’t want to have to worry about exceeding bandwidth on a spinning drive on potentially large sessions.
As for your question on musician monitoring, I didn’t really see your plan for that in your description, is that intended to be happening through the headphone outs of the Saffire?