Hi Fabian,
I wasn't aware of this limitation. Even so you are already live, I would opt for changing this to an item list, especially if this isn't the only case with so many selected items, which aren't even sorted alphabetically.
My, untested, idea would be:
1. Add item list
2. Migrate the data from the choice to the item list using
2a. Migration of the data using the "Upon instance saving" trigger in global actions. Depending on the number of items, you could do trigger the save manually or test whether it's triggered with the "update related workflow instance" in a global action or by REST API
2b. An SDK action in a global cycle, which migrates the data
2c. Adding a path. I'm not sure, whether you can get rid of it afterwards though.
3. Setting the "archival" flag in the "style and behavior" tab for the current field
Archival field - if checked attribute will not be send in attribute list via email, won't render on a form, won't be returned in WebService for mobile devices. Will be shown in history.
Best regards,
Daniel