I quite like [name_f]Theresa[/name_f], [name_f]Theresia[/name_f] seems unnecessary, [name_f]Theresa[/name_f] [name_f]May[/name_f] would be a bad association…
I prefer [name_f]Theresa[/name_f], but both names feel dated to me so I dislike that about them. I’d go with [name_f]Grace[/name_f] for the middle name.