You can check the threads here for various fixes. I've found two different methods that worked for me.
Method 1
https://www.gog.com/forum/septerra_core_legacy_of_the_creator/fix_for_windows_10_works_100/post75 Method 2
After default installation, modify septerra.ini in your installation folder and set the values to the values below and give it a try.
CPUIdle = 0
ScaleMode = 3
(CPUIdle = 0 removes mouse lag, scalemode removes screen flickering caused by fullscreen. Explanations of values are in UOPatchReadme.txt)
The first method has the best visuals and least problems for me, but it does not have the "right click in battle to enqueue character stamina bar" mechanic which comes with the default unofficial patch (installed along with the game).
The second method has that mechanic, but has a little smaller gameplay screen and occasional mouse click issues in the game (nothing too serious).