"The ARM-based CPU manufacturers make this worse by posting almost no low-level documentation for their CPUs."
Are those not based on standard ARM cores or ARM documentation on its cores not deep enough for your purpose?
Are those not based on standard ARM cores or ARM documentation on its cores not deep enough for your purpose?
https://support.arm.com/documentation/uan0016/a/
This has detailed information on latencies and throughput, which are important when optimizing SIMD code. But ARM doesn't publish optimization guides for all their cores.
On top of that, the cores are often modified in significant ways. Snapdragon CPUs, for instance, have used modified Cortex cores in the past and can have performance differences from the original core.
To be fair, Intel's been slacking a lot on this too, not even bothering to update their own optimization guide for their latest cores. But that's made up for the community mining this information in great detail on sites like uops.info, and also being a lot less x86 cores to deal with. On the ARM side, there's practically not much analogous other than the Apple M1 microarchitecture analysis.