I keep getting Payment Instrument not valid when trying to import Contributions - after upgrade from LTS 4.6 to 5.3.x
Ok next thing to try is to check the civicrm_option_value table. Note it has a name column (in addition to what you can see in the GUI in 5.3.x). In the old days this name column was exposed and could be set independently: here it was set to Offline Credit Card - Importing with Payment method:


Offline Credit Card -> works!