This is tangentially similar to what's suggested here
I've also seen suggestions for Draw Order behaving in a mixed/additive way, and I understand why this can't even be defined by the program, as the whole thing is a single list.

However, if "folders" were introduced, they could work as their own key-able internal arrays. The folder/slot group itself counting as a single element of the now "global" Draw Order.
The folders/slot groups themselves could then be keyed in parallel to the global order, folder contents being handled as their own thing and keyed under folder name.
One would of course, not be able to move any slots in or out of the folder during animation.

To be clearer:

I understand it's not that simple to do, and the UI being kept simple and easy to understand isn't only desirable but also a necessity. You've done a great job at keeping everything clean thus far, and I praise you for that. However, I still wanted to bring this to your attention just in case, since I've noticed for a lot of people (including myself), mixing Draw Order keys could make things simpler, as sometimes one needs to clone slots to get around it.

Thanks for the attention and continuous support.