Currently I have to manually categorize as the auto assigned categories are incorrect. This is very time-consuming and it would be great to be able to do it automatically by amount, for split transactions.
Nope. No way to assign categories by amount.
Look in your Tools / Memorized Payee List. If there are entries in the list for the payee name(s) in question, edit these entries. Make sure the categories are correct (or change them) and, to prevent unwanted changes, put a check mark in the "Lock this entry …" box.
