I was reviewing encodings recently and didn't want to drop all the way down to base32, but for some reason the library i was using didn't allow anything beyond base32 and bas64 variants, despite having a feature where you can define your own base.

I thought maybe it was performance oriented. An odd prefix length like base63 would mean .. i think, a slightly more computationally demanding set of encoding instructions?

Either way i basically want base58 but i don't care about legibility, i just wanted double click and url friendly characters.

Yes, the reason is that you need 64 characters if you want each character to encode 6 bits as log2(64) == 6. If you only have 63 characters in your alphabet then one of your 6-bit combinations has no character to represent it.

Base32 can represent 5 bits per character because log2(32) == 5. Anything in between 32 and 64 doesn't buy you anything because there is no integer between 5 and 6.

Now you've got me curious on the performance of base58 to base64 hah. Down the rabbit hole i go. Appreciate your reply, thanks :)