Have you tried the official DE edition Ubuntu distro to see if any of your issues are fixed? My experience is it's pretty good, one of the better linux laptop experiences you can get on a machine with comparable industrial design. Now, perhaps tellingly, I still use Windows 10 on my own 9300. While I appreciate Dell's efforts, I have yet to work with a linux distro on any laptop that does "laptop" things well, like docking/undocking seamlessly, autodetecting docks and USB hubs, having sensible power management settings, etc. So I just run windows now and get my linux fix with WSL.

With my 9300 I use the dell U2720Q monitor (27" 4K with built-in USB C hub). It does 4K@60Hz along with USB 3.1 data over the single USB C, non-thunderbolt cable provided (via HBR3/DP1.4 as mentioned elsewhere) in the box and basically everything worked perfectly immediately and I've had zero issues. Dell/Windows is still "not fantastic" at other laptop things, or at least not as good as apple, but one of the better experiences on the windows side in my opinion.

Relevant to the OP, I would add that when I did go to get a longer cable to connect the 9300 to the monitor, I ran into all of the insane issues described. "WTF, every cable looks identical but support wildly different features???" You have to become an mini-expert in USB technology to know what the hell you're looking for, no idea how non-technical people can even do it.