It’s funny (and annoying) the disparity between Arm builds for macOS and Windows. I understand why it happens but even Microsoft has produced Arm native versions of apps for macOS before Windows.

For vanity GPG keys, I came up with a neat trick where you could insert arbitrary ASCII art into your key. You construct your ASCII art using the Base64 character set. The decoded binary will be gibberish but you can place it in an inert Literal Data Packet that will be ignored by any conformant OpenPGP parser. It just takes a little finagling to align your data to a 3-byte boundary with appropriate line wrapping.

Key IDs are based on fingerprints and fingerprints are calculated by SHA-1 hashing the primary key's public key and the creation timestamp. A computationally easy way to influence the fingerprint is to tweak the creation timestamp which is a 32 bit Unix epoch value. Of course it needs to be in the past so the range is limited but it's faster to do it this way instead of recomputing the cryptographic key.