This I agree with, deploying and running code for the correct cpu is a problem with no established solution.
As for actually writing the code, portable_simd is great. You need to adjust simd width and compiler config for the cpu you deploy to and fill in the blanks with intrinsics. Which is much less work per target than writing it all with raw intrinsics if you are deploying to more than one target.
https://github.com/seclorum/SIMDSynth
(uses SIMD for optimizing voice allocation and filter calculations on both ARM and x86, same codebase ..)
I had a quick glance of the code and it seems to use mostly basic arithmetic instructions on fixed width simd vectors using some helper macros like SIMD_MUL(x, y) for _mm_mul_ps, etc. Some explicit simd intrinsics code for stuff like sin.
That would've been pretty easy to write with portable_simd in Rust or the equivalent C/C++ language extensions (or maybe C++26 std::simd).
You'd just use f32x4 (or add a typedef with attribute in C++) and then use x*y instead of SIMD_MUL.
For basic stuff like this, you should get the exact same generated code.
https://clang.llvm.org/docs/LanguageExtensions.html#vectors-...