> It would have been expensive, but all characters should have been fixed size 64bit values

You're making the same mistake that numerous people made before you: thinking that it's as simple as using arrays of large enough numbers. First they thought that two bytes per symbol would be enough, then four. Spoiler alert: it wasn't. And eight won't work either.

Why wouldn't 8 be enough? Surely 18,446,744,070,000,001,024 characters is enough for every writing system in the world.

UnicodeV6 - 128 bits per character!