Strawberries can be quite productive.
So can grapes. If also seen monster harvests of quince. But dunno if your area is fire blight prone. However, as far as im aware apple is one of if not the highest yielding fruit per surface. (in current comercial production) surpased by tomoto if you consider that a fruit.
I think however it’s an unfair comparison if you compare something low vs something high. The shadow the higher bush will trow, might decrease the yield of the next plant due to shading.
If you take shading into account. And don’t mind shading of walkways. you could use http://shadowcalculator.eu/
If discribed how to use it in more detail in this post.
If used it to determine the shade length a tree gives. For me for a east west row, the shade length is approx the tree hight. So im makeing that my row width.
Another way to increase yield is planting something that loves full sun. Like an apple tree growin in https://apal.org.au/programs/future-orchards/future-orchards-library/ style.
And plant something like goosberries that tolerate the semi shade that tree creates in the shade of that tree. This will likely increase effective yield per sq feet.
Although i like productive fruit plants. I think a more intresting question is how to maximise yield, for most fruits.
I think maximizing light interception and airflow. Minimizing unwanted shading and disease and unwanted overly vigerous growth would be an excelent way to go.