One thing for the tank top bag consideration is sun and phone. I put my phone inside the tank bag vs it being on handlebars. This helps my expensive phone to be kept from being blown away and not break the camera in it with road vibration.
When riding in the sun, I am in S. Florida, the black bag gets hot inside. In addition, I am running wireless Android-Auto from my phone, and my phone does do a slight CPU workout. But even if it didn't, the bag can still heat up so much, at times the phone would display that it is shutting down due to reached temperature. I have to open zippers slightly to allow some air exchange. In the top clear layer pocket where a phone would usually go, I have put a paper map. Not that I really need a map, it is just good at being not black as the pocket itself, reflecting some sun away, preventing the top of the bag heating faster. Plus, a map kind of does not look out of place. Maybe I look like a tourist, I donno.