Onward to answer my own question.....Actually, the current version of Luna will do exactly what I was wondering about. It will connect directly with a Thunderbolt 4 cable and the latency and delay is literally not noticeable. Currently Universal Control is very very wonky on anything past Sonoma 14.3 and has not been fixed as of Sequoia 15.1.
You do lose the stereo speakers but this is easily fixed with a set of headphones. I have not heard from Luna about whether the SSD on the MBP shows up on the. desktop, but a 120Gb external drive certainly makes up for that with those speeds on the M4pro Mini.
As to being happier, with the an apple quality monitor, new mouse and keyboard I’m not so sure. I would have to leave to living room where I usually listen to music and go to a room far away, as well as sit in a chair, not the couch, and be away from the warmth of the wood stove(I live in Maine), and the cost would be up near the price of just buying an new M4Pro in the macbook or about $3900. I figure the cost of the M4pro mini with the 14 core chip, 48 GB ram, and 1TB SSD plus the $80 Luna, plus the cable, plus an external drive to be about $2400.