I am still wondering what you mean by 500+ lines of code for a merge.

do you mean the list of columns which should be updated?

If you want to update a subset of columns that can become cumbersome indeed. But with some coding in scala/python you can create a list of column names which you can then pass to the query.

If you want to update all, use *

MERGE INTO table 
  USING updates 
  ON mgline.RegistrationYear IN ($yearlist) AND
     table.key1 = updates.key1 and 
     table.key2 = updates.key2 and
     table.key3 = updates.key3
  WHEN MATCHED THEN 
     UPDATE SET * 
  WHEN NOT MATCHED THEN 
     INSERT *