I guess it depends on what your requirements are. I’m not too knowledgeable on the pricing side but your price increases/decreases as you noticed aren’t linear and vary based on the amount you’re buying or selling. Buying or selling currencies from a trading partner exposes them to fluctuations in that currency so they’re going to need to be compensated to cover their trading costs like hedgeing (if your transactions are large enough) as well as some sort of profit for them.

If you’re just doing it for a retail perspective some sort of averaging scheme like you mentioned would probably be okay. If you’re dealing with large amounts regularly you’d probably want to negotiate with multiple liquidity providers to get an idea of what kind of rates you’d get for a given currency pair.