Do a roadmap. Fixed scope small project that you get paid for where you define requirements. Deliverable is a functional requirements doc. It derisks the project for them, and gives you more confidence when quoting a fixed scope fixed price project. You just don’t guarantee timeline. I’ve been doing it for over a decade - it works wonders.

What is the total cost of these projects and how much are you typically charging for the requirements doc?

I'm skeptical that most of the clients I work with would go for something like this. Generally my projects are in the $7k-20k USD range (though with some clients the total spend is much greater with additional phases of work...)

Also, what do you mean "you just don't guarantee timeline"? Why not?

Best way but hard to find clients open to this as it requires more trust especially if it starts to drag on.