I encountered most of these issues, with the additional one that some games would force themselves into 3840x2160 regardless of scaling, and then run terribly.
Eventually I just bought a ~2008 30" Cinema Display and have been incredibly happy with it.
30" Cinema Display is a joy. I used to have one but such a shame that it runs too hot. Living in the UK and working in an office with bad AC, it melted my face :) I let it go.
To anyone planning to use a 30" CinemaDisplay , you will probably need a special Mini DP to Double Dvi active powered adapter. They are not very common so they are a bit expensive. Search for: Tripp Lite Mini DisplayPort to DVI Adapter Cable with Dual-Link Active USB Power MDP to DVI-D, 6 in. (P137-06N-DVI-DL)
MacBook >
Thunderbolt dock >
USB-C to Dual DisplayPort >
2x DisplayPort to Mini DisplayPort >
2x Mini DisplayPort to Dual-Link DVI >
2x Cinema Displays
Eventually I just bought a ~2008 30" Cinema Display and have been incredibly happy with it.