Your downside is a major concern, but it would take all of 5 minutes to patch sha256sum to have a new output format, and another 3 months to have a PR approved. Ditto for most libraries, except where written into standards (and I'm not sure if that's anywhere). I'd expect Python 3.9 to not be a problem either. So it's not a big deal to address the concern.
If you want to do this well, though, the trick would be to come up with a human-friendly format. You can't use both '0' and 'O.' You can't use '1,' 'l,' and 'I.' Invest the time to do HCI work.
And, ideally, you'd have some form of internal check-summing, so you can easily flag most typos.
Performance is NOT a concern. Some folks here mentioned the some encoding formats are table lookups. In most cases, generating or verifying a cryptographically-secure hash is many orders of magnitude slower than an encode/decode of the key into any format. And if you're not generating/verifying, in most cases, it doesn't make any sense to encode/decode (just pass around the encoded key).