@li-zi i know you kind of solved this for your case already, but it seems to me that one other potential solution is to use the Custom Wizard plugin.
I use this to prompt new users to fill out the wizard after signing up.
The wizard has some fields that offer dropdown options.
Then i set up two separate actions to complete after the wizard is submitted:
- one to update the user profile custom fields with the wizard field from the drop down
- another action to add user to specific group based on the wizard field selection