Combine color levels with indexed conversion with single preview

[bicyclesonthemoon]

Thanks again for answer.
I checked today the Gegl graph option,
this is an useful tool but not in this case, as converting an image to indexed is not a Gegl operation.

So I would have to look into the plugin option.

Is there a place where I can ask to hire someone who would be willing to make a plugin?

What I would like to have the combined options from both dialogs, with preview.
as shown:
(I marked which parts are necessary for me)