The fundamental mistake is in trying to take input for one purpose and transform it for another purpose. Just have the user fill in an additional field for their name as it appears on bank statements, or whatever the second purpose is. Trying to be clever about this stuff never works out.

What you call the second purpose is often the only purpose. Or you have to talk to half a dozen other systems each of which have different constraints. You wouldn’t want to present the user half a dozen fields just so that they can choose the nicest representation of their name for each system.

That being said, in Japan it’s actually common to have two fields, one for the name in kanji (the “nice” name), and one in katakana (restricted, 8-bit phonetic alphabet, which earlier/older computer systems used and some probably still use).