Hello @alexcr
I use my MacBook Pro mid 2010 with High Sierra and KM 10.2 connected to an Apple Cinema Display 27“ with either one of these Macros by @_jims:
and with the little MenuBar Application DisplayBuddy with either the display synced to my MacBook Pro or otherwise - and I have no issues.
There are more ways to do this but I think that’s one of the easiest 
Hope you found this useful.
Let us know about how it goes - and have a great Sunday.
Greetings from Germany
Tobias