I think that it’s better to keep your clients on your own ads manager. They can add you as editor or admin and then you can run ads from your facebook ads account, as you are already doing this for some of the clients. This way you have control of everything.
Give a price on condition that its for minimum 3 months, 6 months, etc.
Or, give a monthly price, and have an additional Set-up Fee for first month. This is reasonable because first month is where majority of work is. And,
Also, sign an agreement with a minimum notice period for termination. (these are hard to enforce if they break it and refuse to pay. But most people will honour it unless you have done way less than promised)
Build a good business relationship. Keep the communication going.
If they question effectiveness of campaign, highlight that it takes 2-3 months to test and tweak the ads to get proper results, which is the truth.