Not a PIA at gas stations, especially the tank lock bags. The purpose of a tank bag IMHO is to have easy access to small item things or frequently used, the bigger they are the more cumbersome, heavy and annoying they become. I do like some of those crash bar bags like you mention, that seems like a really good solution and option for many.
This^
I started with a larger tank bag 15 years ago. Too easy to put too much stuff in them. They are NOT cargo bags! Since I don't read maps while riding, I don't need that detail. I do often put a route sheet to make sure I have taken the right turns and such that I can confirm with a quick glance.
Think of the tank bag this way: it is a large wallet. What do you need when you go into a store after fueling? What do you need when you go in a restaurant to eat? What do you want with you when you go into a restroom at a rest stop?
My wallet, phone, ear plugs, note pad, glasses are mainly what is in mine.
With the quick release tank lock I flip a lever, disconnect the tether ( optional but I do run one after having magnetic bags fall off...) and off I go. You want it small so you don't have a big piece of airline luggage to carry into the store. I like to leave my shoulder strap on mine, handy to throw over the shoulder so you can shop, pay, and do restroom duty.
More stuff....that goes into the top case or dry bags on the seat!
Water issues? They don't like rain beyond a light shower...but put the cover on and they are all day rain proof.