Extremely limited is my middle name! :D Joking aside, I’m sure you can figure it out - other people did before :)
I have no idea how Factorio does conveyor stuff, but I think just allowing straight lines (like in OpenTTD) would already improve the UX a lot, and in this case you don’t need pathfinding, continuous polling of the mouse position and stuff like this, just a start and and end tile and a “for” loop to cover the tiles between them with conveyors.