Wanted to provide an update in case anyone else comes across this post...
As I mentioned, completely starting from scratch and creating new feature layers was a possible workaround, but the amount of extra work in regards to data management and how it relates to another application we use would have still been pretty tedious.
What seems to be the best solution for me, given my restrictions, was to edit the JSON directly for the feature layer. (don't know why I didn't think of it before!) What I did was manually edit the choice lists just for one layer using the AGOL GUI. I then went into the JSON code for the layer by going to the REST endpoint for the layer and adding "admin" to the URL between "rest" and "services" (see screenshot).
I could then copy the section of JSON I needed for the domain values and paste into all the other layers.
An important thing to note (this is the way I was taught to do it anyway) is that if you edit the JSON like this, you need to change the lastEditDate value. Delete the numbers and add empty quotes "" before clicking "Update Layer Definition".
I'm sure there's probably still some more elegant way to do this, but this solution worked for me so that I could avoid having to publish new layers.
Best,
Katie
“The goal is not simply to ‘work hard, play hard.’ The goal is to make our work and our play indistinguishable.”
- Simon Sinek