I know it's been a while since you've posted but this is for reference for folks. I can't speak to how to easily make the values alphabetical in the feature layer. I'm not super skilled at coding so the one provided doesn't make sense for me in the AGOL environment.
Since it's primarily for the field folks, I'm making the assumption that data is being collected using Collector, Survey123 or Field Maps. Perhaps Survey123 would be worth considering the most.
My first suggestion doesn't lend itself well to workflow, but if wanting the initial change up to alphabetical order: export data back to geodatabase. Create the domain with the values in alphabetical order using Excel then table to domain, and reupload as new hosted feature layer. This is not great if you have this layer on a bunch of maps. But the values will be in ABC order.
The second is creating a survey using the existing feature service. You'll need to use Survey123 Connect which allows for creating a survey in Excel. Updates to choices is pretty easy when the feature service is published again. The survey can be created to depict the choices as a drop down in that's still searchable in the field but will be in the order that has been set in S123 Connect. With a list that's super long you'll want to use linked content-csv instead. You might use pulldata or external choice lists. Check out this link by ESRI that leads to explaining more.
The quest continues otherwise to figure out how to get these rearranged behind the scenes on the actual feature layer for when working at the computer in AGOL or Pro.