I just ran across this and noted that, for mainly heating useage, he puts his inside unit at about 18 inches off the floor. This is the first I had heard of this, but it makes sense to me.
https://www.buildinggreen.com/blog/7-tips-get-more-mini-split-heat-pumps-colder-climates
https://www.buildinggreen.com/blog/7-tips-get-more-mini-split-heat-pumps-colder-climates