You can’t ask for an increase in spend everytime the clients adds a campaign or an ad group. At the same time, the client won’t reduce your retainer if he pauses few campaigns and ad groups, unless they shut completely a platform. It works both ways. Otherwise, just ask to be paid hourly.
I like to do flat monthly rates as well, and just try to be as clear as possible in advance about the scope of work so there is minimal worry of creep. For my largest clients I do like to work out some kind of performance bonus structure, but otherwise I like the simplicity and reliability of flat rates. I get pretty good close rates, so clients seem to like it too. I don’t have any set pricing table but for every contract I will negotiate at least one increase in fee at a particular ad spend milestone.