Seems like setFieldOptions() method does not work on the assignee (or any user fields) to limit or restrict the users available in the list.
Is there any other way to achieve this using behaviours?
Not that I am aware of. I think in big part because of how this field behaves (dynamic rest call rather than pre-loaded options).
The best you can do is validate the user input after the fact and set an error if the selected user does not match your rules.
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