The "when to throw out an estimate" is something I've evolved on a lot in my career. Initially, naive happy path. After that, happy path + some overly-large fuzzy padding number.
Now, I push back on PMs who ask for an estimate at an unknowable point in the project.
The problem is not estimating work. The problem is estimating work volume risk (the range of work the project might take). And the only thing that can decrease that is prototyping and similar.
I work in somewhat of a sub-niche (automation), so have a larger than average number of black-box, outside-of-my-control, unchangeable components. But I've come around to believing modified spiral [0] is a better approach when risk dominates.
Sometimes, the right answer to "How long will this take?" is "It will take this long to get an answer to that question."
[0] https://en.m.wikipedia.org/wiki/Spiral_model