E.g., can they assume that names can be expressed as a sequence of (current) Unicode characters with some specific maximum length? Can they assume that names have no leading / trailing spaces?
So, you can make reasonable assumptions. What is reasonable will change, which is fine because the way courts figure out what's reasonable in some particular case is to either have the judge decide, or have a jury decide, and people change too.
The nice thing about reasonableness is that you are equipped to make a first pass at judging it yourself, since you are presumably a reasonable person. If you need second guessing, have a team mate consider it, and, if you're worried that your collective idea of "reasonable" might be distorted in an important way, that'll be why your organisation probably encouraged diversity to avoid that.
You might say, this seems awful because it isn't precise enough to say, implement it as a Javascript library. That's true, but intentional. Justice will necessarily involve such judgement calls, and trying to evade that by specifying everything precisely with no room for judgement is a bug not a feature.
Regarding trailing spaces etc, IMHO the standard would be "as shown in passport" i.e. trailing spaces definitely would not matter, but spaces and punctuation between words would (e.g. D'Artagnan as a name). I looked for but did not find any specific restrictions on name length. In general, the country will have regulations on what they accept as names in their official IDs, and again you may piggyback on other institutions - as long as you accept everything for which your government have issued documents, you should be fine; and if someone has an interesting case that requires changing the process, let that fight happen between them and the government first.
If you want to be safe maybe allow for up to 1000 arbitrary codepoints? :)
I mean something like "${name} spelled with an acute accent on the e" would be technically a correct description even if it is impractical to use. The GDPR does grant you the right to correct your personal information but doesn't specify how this information is represented.
As far as I can tell the GDPR also doesn't grant the customer the right to have their name represented correctly on their bank pass (otherwise everyone with a long surname would require impractically long bank passes), the court only ruled that the inability of the bank to store the name correctly simply isn't an excuse.
If you did officially change your name to something interesting, I presume the bank would definitely have to accommodate it; but the restrictive part would be the process of actually changing your name.