Why not get the payments for helping them define the project requirements, and then if you still smell boondoggle, you can quote them an estimate that you can actually profit on but may make them run to a competitor for a lower bid. Now your calendar is clear for projects you’ll actually make money on, plus one of your competitors now can’t compete as well on your next bid.
But he ran into the same problem: an explicit discovery phase gets summarily rejected. Even though it would benefit everyone. I suspect deep down they know what they’re asking and they want to trick you into saying yes and getting sunk cost fallacy before you sober up and start telling them no.