The factory can help with a lot of this (if you incentivize them appropriately).

The factory can never help tweak your initial concept, right at the beginning of development (when it's ~free!), to save tremendously on future production cost. That's what you hire the consultants for.*

*Disclaimer: I'm just such a consultant, though I work on the EE side.

This is critical and underappreciated.

Numerous small design choices early on can have drastic impacts on the complexity of the mold and how expensive it will be.

A simple example: designing your side I/o ports to be on the parting line, or move the parting line up through the hole centers. Didn't think of this? Now you need a slide.

Deciding the surface finish you want also impacts hard it will be. You can hide a lot of flow problems and tooling marks with a nice rough textured finish.

Some colors show cooling and flow worse than others. Clever gate placement hidden under something hidden in assembly makes cooling simpler. Etc, etc

Well worth the money spent on a guru to look at your design and provide feedback. (I am not one, I've just hit every wall)

It is available sometimes (at a price). There are some upsides, namely in-house experts are more familiar with that specific suppliers processes and capabilities.

Downsides are obvious, they work for the supplier and not for you.