He also said the motherboard looked like it was in good shape
That just sounds like he did a visual inspection and didn't see any scorch marks or cracked soldering joints. If the problem occurs with two chargers, two batteries, and the connector has been replaced, then to me, that leaves the motherboard. And with that, the tech may be right about not spending any more money on it. But you sure don't have to buy from him.
Your only other option that I see is to take it a different shop and see what they say.