Thanks for the details, indeed the fact that the option would immediately disappear is a problem!
What i would see as a good and easy way:
When clicking the +, instead of directly adding the value in the referenced table, the “card” modal would open, so the user can fill in the other fields. Same behavior as when clicking the link icon for editing.
I actually think it would be useful to always have it, even when there’s no dropdown condition, so you won’t have to go to the referenced table to fill in the other fields.
On a UI level, I think it would benefit to not appear as big, and without the top bar menu and cross, something like that:
wdyt?