I used "Automatic Belt Planner" to do this sort of auto-routing in the past, but it's not much maintained, so I'm excited to see someone new pick up the torch. One thing I miss, though, is that ABP had a nicer -- and more "human visible" UI for configuration, and for letting me confirm the proposed path was what I wanted.

It'd be great to see those adopted. The specific elements that I miss are:

The configuration GUI after placing their "start" entity in the world. This allowed selecting surface vs underground placement modes. That was much easier to work with than the different shortcuts for different modes. I keep forgetting which is which.

It drew a proposed line in the world, showing where it planned to run things, and I could accept or reject that. That made it much easier to do semi-automatic routing.

It had a "after you place the ghosts, start another route from the end-point", which also made semi-automatic routing much easier to do.

ref: https://mods.factorio.com/mod/beltplanner-patched-18

I'm excited to see someone new pick up the torch.

Thanks a lot for the support :D

ABP had a nicer -- and more "human visible" UI for configuration

I indeed feel that the current implementation is a bit too vague and hard to work with. I'm still thinking about possible ways of presenting user with a friendly UI. I'd love to hear any suggestions about how to get that done.

Just share some of my thoughts:
My first step of drafting the UI is to split up the user interaction flow to 3 steps:

1. select a starting belt
2. select a ending belt
3. select routing mode

And there are other functionalities I want to support, like:

1. select a batch of starting belts
2. select a batch of ending belts
3. select routing mode

And as well as the function you've mentioned:

It had a "after you place the ghosts, start another route from the end-point", which also made semi-automatic routing much easier to do.

I would describe the process like this:

1. select starting belt
2. select middle belt, and possibly repeat this process
3. select ending belt
4. (select routing mode here? or before select starting belt?)

My second step of drafting the UI is to give a meaningful UI for each of the step described above.

For selecting starting belt(s), I might add a selection box indicator? Or a belt queue at bottom-left? Or both?
For selecting ending belt, since this selection will be immediately routed, so no indicator needed.
For mode selection, I might add a UI box at the center of screen to ask for routing mode?

But above solution doesn't solves last flow: "after you place the ghosts, start another route from the end-point" and there is no terminating condition yet.

Would love to hear any solution for that

I like that it doesn't require any GUI interaction. Just copy paste from start to end. That's great! Having an optional preview of the result could be nice if I can skip it. Hope Wube adds some API interface to make Ctrl + Z able to undo the ghost actions of mods.

Also I would switch alt + left click with left click. Makes most sense as the default to have shortest route with undergrounds where necessary, a balance of both types without excessivly leaning into one type. It's great that those are options, but they should be with the modifiers. Thankfully I can switch the keys to the "correct ones" by myself. But they key description for the one that prefers underneathies doesn't mention that, which makes it a bit confusing.

Having an optional preview of the result could be nice if I can skip it.

Good idea. BTW do you like the current way of displaying starting points?

Hope Wube adds some API interface to make Ctrl + Z able to undo the ghost actions of mods.

I think there might be a chance I can support this by faking the ghosts placements as player placing blueprint. Will test about its possibilities.

Also I would switch alt + left click with left click. Makes most sense as the default to have shortest route with undergrounds where necessary, a balance of both types without excessivly leaning into one type.

Good suggestion, but in considering that this is configurable, I'll keep the key map for now. (Plus I often prefer mostly underground since it leaves a lot empty on-ground spaces for future xD )

But they key description for the one that prefers underneathies doesn't mention that, which makes it a bit confusing.

Emm indeed, will update the description.