The hell are you saying? Sterne isn't limited, his banner doesn't matter. He can keep farming Sterne's shards via his hard quest.
The better advice is to pick one UR to focus on first if you're F2P because of a lack of rainbow spheres. If you're focusing on Frederika (who is a better option than Sterne), then Gaff is a great alternative to Sterne. So yes, go Gaff if you must.