Comment by cube2222

I think the main problem in estimating projects is unknown unknowns.

I find that the best approach to solving that is taking a “tracer-bullet” approach. You make an initial end-to-end PoC that explores all the tricky bits of your project.

Making estimates then becomes quite a bit more tractable (though still has its limits and uncertainty, of course). Conversations about where to cut scope will also be easier.