anton

You could save 0.64 bit per char more if you actually treated you output as a binary number (using 6 bits per char) and didn't go through the intermediary string (implicitly using base 100 at 6.64 bits per char).
This would also make your life easier by allowing bit manipulation to slice/move parts and reducing work for the processor because base 100 means integer divisions, and base 64 means bit shifts. If you want to go down the road of a "complicated" base use base 38 and get similar drawbacks as now, except only 5.25 bits per char.

Unless you only copy and compare you have to decode it, or implement more complicated logic for everything from searching to concatenation (which is normally just memcopy).

I can't comment whether learning C first improves your rust, but it certainly makes you appreciate what the rust compiler does.
Also learning rust improved my C.