The base Rust experience is so nice that it ends up spoiling you :).
Day-to-day ring development happens on Windows and it was designed to "just work" on Windows (as well as Linux, macOS, Android, iOS, and other supported target operating systems). We did a huge amount of work to make the build system fully automatic to enable this, and we have a pretty extensive CI mechanism to make sure everything "just works" as much as we can.
As for WebAssembly, I'm looking forward to something like https://pdfs.semanticscholar.org/3887/6d86e5e7851181efc9ed3b....
However, you should probably use the WebCrypto API in a web app if at all possible, instead of a wasm crypto library. Unfortunately the WebCrypto API is 100% asynchronous whereas almost every other crypto API is 100% synchronous so that's easier said than done. Really browsers should add a synchronous WebCrypto API; then crypto libraries can transparently delegate to it when targetting wasm.
async just makes asynchronous look like a synchronous code, but it's still event-based and async.
Yes definitely. Crypto libs are so critical that building on something like BoringSSL is still good win. There were attempts made to build pure rust crypto libs, some of them are not maintained and some are a bit granular. I think pure rust lib similar to BoringSSL is going to be quite an effort