I guarantee you with my lack of skill, my attempt at using portable simd will exist while using manual intrinsics I doubt I'd get there.

The question for me is whether portable simd will result in faster code than plain auto-vectorisation; for the simplest loops auto has me beat (the few times I've tried it), but I imagine as the complexity grows I'll be more likely to try do something that breaks auto-vectorisation, and it'll be more obvious to me when I do that in portable simd.

nowadays it doesn't matter if you personally don't have this particular expertise, people still expect you to use an agent to write this code

Portable SIMD is a bad abstraction. The only two good choices are "something like ISPC" (basically "autoscalarization", similar to GPU kernels) and "writing in assembly" (ffmpeg does this for a reason).

Note that ISPC does not handle symbolic math e.g. sparsity preserving linear solvers or equation simplification a la Mathematica. Manual ASM does. ;)