What a rollercoaster ...
thank you very much for the time and effort you put into experimenting regarding this issue.
Unfortunately, I don't have a screen available that supports higher refresh rates than 60 Hz, but on my 2016 MacBook 12" I am getting the same bad results "solo" as when having it connected to a bigger screen (which is also locked to 60 Hz).
I did another test using a 50p video, to rule out the "internal screen 60 fps limit", but the playback is also very bad. It looks like in Davinci the MacBook's performance allows me to go Full HD with 30p while keeping very smooth playback quality. Everything above hits a wall. Tomorrow I'll have a chance to do these tests on a 2019 MacBook Pro 13".
The strange thing is, that 1080/60p video playback in other programs works fine on said 12" MacBook, without tearing, stuttering or any other unpleasant occurrences. It seems like Davinci Resolve Studio is using a different method to synchronize the visual content to the screen. I'll drop Blackmagic support a message, hoping that they can help me with this.
btw: This was tested on macOS Mojave.
Best regards