Alas, I just started working on a Rust wrapper for Botan https://crates.io/crates/botan
The existing Botan C API is in fact sufficient for OpenPGP already, https://github.com/riboseinc/rnp is in C++ now but was originally C and uses Botan's C API.
But Nettle is IMO quite solid and the developer is very skilled, so full steam ahead.
Is there any relation between Sequoia and the BoringPGP spec?