On the project at work, we'd like to create a custom tool that would get a HDA and process it without having to drag the object in the scene manually and going through the cluttered menu. I've been looking for hours online for an example of a custom UI made in Unity to process a HDA and I am surprised that all my searches are returning nothing useful.

The default HDA inspector has tons of buttons that we won't be using that are just increasing the risk for user errors. We'll be using this HDA to create geometry to be used with Particle Systems, so we don't need it to exist in the scene.

I'd like a very clean UI in its own window and for the user to be able to set everything up before pressing a button that would then process the HDA and prompt the user to choose a save location for the resulting FBX.

Is this even achievable? If it is, would it be possible to provide an example project? I just think that it would be very useful...

As a bonus question, would it also be possible to control the exported mesh's Import Settings? I need "Weld Vertices" to be off specifically for those meshes, but I don't want to use a processor that will affect all meshes in the project.

One way is to use HAPI directly. In fact, HEU is built on top of it. But you would have to do a lot of heavy lifting by yourself.

What we do is to write Unity components that are added to the HDA root object and then use the HEU parameters API to control the parameters of the HDA. You could additionally hide the parameters from the HDA to not expose them to users.