Fixed pricing can be fine, but requires a lot of due diligence with both parties, and much more detailed contracts and crucially – scope. Expectation management is key. (Btw, what you’re describing sounds more like a retainer which isn’t quite the same.)

My experience is that fixed price only works for very short (days and weeks, not months) projects that can be scoped very strictly. Even then, there’s a lot of risk of miscommunication and uncomfortable discussions. My rule of thumb is that if I’m doing a fixed price contract, it can’t be longer than two weeks. This usually amounts to very targeted fixes, upgrades, research projects. Things that are typically not so hard to time box.