Is there a way to mass update the item pricing sublist specifically for items with custom price level only? I've tried to start from a saved search on the customer record but i can't seem to get the sublist results I'm looking for (even after narrowing down to 1 item only).

yes you can. You can find the steps in NS documentation under this topic "Updating Item Prices"(/app/help/helpcenter.nl?fid=section_n2184312.html)