I know that it may be a pain to do it, but be able to change the order like dragging the combinator up or down, or simple 2 buttons (move up, move down) that move the combinator in the list would be very helpful and would speed up the processes.

Another thing is to be able to duplicate a combinator, with normal combinators, I usually just copy the last one and change one thing, at least most of the time. Been able to add a copy of a row and than change it up or down in the list would be very helpful.

Hi Sworn,
Dragging objects is restricted to children of the LuaGui::screen (i.e. the main dialogues). Adding buttons would work, but it would require the entire GUI to be updated so they can be fitted in. It would be a nice quality of life feature, but it's just going to take too much time to implement.

With regards to your other suggestion, I think it would a nice feature and should be quite easy to implement. I'll look into adding it the future

It turned out that, it's currently not possible to copy settings from Factorio core entities (i.e. Arithmetic & Decider combinators). Unfortunately that makes it impossible to implement your second suggestion.

O damm, I suspect that cause you can't do it with trains. But anyway, thanks for looking this up.

On this QOL topic.
1) How about add new combinators to top of list, not bottom?
2) You have input count. can you add "Output to first item in test condition" (but with better wording) i.e. decider, IRON < 200, set to first (IRON) or autoset last item on first item choice?
3) Can you set the item chooser to remember the last category you chose?
I will make a new thread if you think this is better

I'll attempt to answer your questions.
1) The mod's UI is inspired by the Train Station UI and it was designed to resemble it. That's why the new items go at the end. Changing it now would require quite a lot of code changes.
2) I'm having trouble understand your point, could you go into more detail
3) Yes, that is possible and it should be quite easy to do. I'll add it into my future TODO-list

expanding on 2)
given we want a new combinator like...

Iron Ore < 200 output Iron Ore, which seems very common,

a radio button like 'input count' that just set the output to the first thing selected in the combinator (Iron Ore, in this case) seems like it would save a third
trip to the chooser.
Perhaps call it 'Input Item' ?