some people don't even have a native writing system so i guess their name is not canonically mapped in unicode characters either. every attempt to write it down is just an approximation.

That's just awful security practices. You must not replace proper code/data separation with user input sanitization. If you are just pulling names from a database and inserting them into your DOM directly you're doing things majorly wrong and half your codebase probably needs rewriting from scratch.

If your stack does not support code/data separation, you should escape at the point of use/point of interface with other software, not at the point of user data entry.

You should not "sanitize" something as personal as a name. It is up to the individual to identify themselves as they see fit, whether it is some weird legal name or just how they want to present. For every "rule" about names you can think of, there will be an exception somewhere.

In fact, even splitting up the name field into "first name" and "last name" is already wrong. It should just be "name". If you need there to be a separate "first name" and "last name" for some reason (e.g. an external system which requires it), allow leaving either one as empty. Bonus points if you have independent "legal name" and "how would you like to be called" fields.

Getting rid of excess whitespace is fine aswell as it does not change the name

As a responsible developer you must not assume this. Especially if your software interacts with other systems. You never know what dumb shit some other system has got up to, maybe a clerk somewhere accidentally entered someone's name with a space and that person desperately needs to use your software while they're getting things fixed.

As an immigrant, I have been personally strongly inconvenienced by user input validation very often. For example, a tax agency system (which has my passport number recorded with a space) rejected automatic declarations from my bank (where the system did not allow spaces in the passport number) so I had to fill my tax declarations manually for a while. Or my bank rejecting bills from the water utility because the utility's system required entering two surnames, and I only have one, so they just put it in there twice. The amount of services which reject my pretty normal-looking self-hosted email address with "enter a valid email address" (presumably it must end in @gmail.com or @outlook.com) is staggering. This kind of bullshit is widespread and it needs to stop. You as a developer don't know better than the person entering the data about themselves.