This is the import template that i am using
Hello Stefan,
First, thing i notice is the year column which is in your excel a string value i think and in your model an integer.
The rest looks quite fine to me. What i always try to do when I import an excel is to first import it one on one on an entity that just needs to capture the data; so i do not really use the association and key option, after i upload it i retrieve my imported captured data objects and then execute the mapping and the logic with microflows.
This also works well with debugging the problem.
Hope this helps
Good luck