I just tried to use my 4-year old iMac Pro as a (5k) display for a new MacBook Pro 16. The MacBook is roughly twice as fast as the iMac.
Unfortunately, it looks as if it is not reasonable to reuse the iMac as a monitor for the MacBook. Extending the display was not very difficult settings wise. However, the mouse is very slow and the display quality is lousy. I am enclosing a photograph from an iPad (which is a little overexposed, and actually makes the MacBook look worse; the iMac looks representative). in the photograph, the MacBook on the left is driving the iMac on the right. the Terminal text is on the same settings on the left and on the right. the connection cable between the two is Thunderbolt 4, though I am not sure whether it is automatically chosen. (they could try to tunnel differently over the network somehow.)
unless someone has some good advice how to drastically improve how to airplay share a display, I would not consider such a setup for primary day-to-day work. even a 1080p monitor will function better. so, is there something I am doing wrong?
I will probably buy a 4k 27" monitor for $400. The LG 5K is a ridiculous 3x price, the Apple XDR 6k is a ridiculous 15x . Factors of 2x and 8x would have been more digestible. With the saved money, I will probably be able to buy an iMac 27" with M* processor when it finally appears...