"It does require an extra click to open, sadly..."

I come to the conclusion, that I really hate that "extra click". Is there any way to get the plate just open the icon selection when clicking on it without that intermediate UI?

If not, is there any way of migrate back to IndustrialDisplayPlates mod without loosing all icon assigns?

As far as I know there is not a way to directly open that UI. If you can find one I'd be happy to implement it, but I've been trying to find ways to open the icon selector directly a few times in the past 😔

As for migrating back, probably not.

This mod does a "migration" when started beside IndustrialDisplayPlates to re-create the icons rendered in the world (on top of the plates) and then destroys the previous render by IndustrialDisplayPlates to not have duplicate icons floating on top, and then you disable the old mod. You could try enable both side by side and disable DisplayPlates and see what happens, not sure of the outcome there but both mods should use the rendered icon as the state for which icon is selected.