Trying to jam in the extra characters for "efficiency" also doesn't really do much. Base84 is 6.4 bits per character, base64 is a flat 6 bits per character, base58 is 5.86 bits per character. It may intuitively feel like having 26 more characters to encode with would be hugely more efficient but for the extra risk of what all those characters get us we're only getting just over .5 more bits per character. I need to be either pretty desperate for efficiency, rare in 2026, or very, very sure that those things are only going places where those characters aren't risky or inconvenient, which is pretty difficult to guarantee since even if it is in fact true today who's to say it will be tomorrow, for this to be worth the risk of the extra characters.

Even just the extra characters in base64 are often annoying and we already have multiple common base64 standards because even trying to find just two more characters to fill out a-zA-Z0-9 is hard.