I like them both and think they’re very cute. All the spellings look fine to me but I do prefer the ie spellings. Imo Millie/Milly works well as a standalone name but Tillie/Tilly should be short for something.
Marilla &
Amaryllis could be fn for Millie/Milly. For Tillie/Tilly, Otillie or
Tilda, maybe
Twila?