Momentum works well so long as you have enough alignment, clarity, and certainty.
When you don't know what you're building though, the last thing you want is a heads down project that you can't change course on or kill because it has too much momentum going in the wrong direction.
Whiteboarding or taking notes can be a good proxy for thought work in early stages. Iow, those are good ways to "get something imperfect out" early because they are at a high enough level that they don't kill much time. However, hacking code out before it's been thought through tends to be a massive time suck.
I definitely agree that there's a shitton of value in hitting things regularly though (usually daily).
And remember, you win by showing up each day, not by tilting at windmills.
If proceeding requires a review/input from someone, its a difficult balance between respecting their busy workload may mean they only get to it next week, or letting momentum die.
Re AI: I find that Claude can massively help and massively hurt with this, the devil is in the details.