There are some NAS models with Arm-based CPUs and multiple SSDs/HDDs, but those have a very low throughput due to using e.g. only one PCIe lane per socket, with at most PCIe 3 speed.
Not really.
ARM (and to lesser degree, RISC-V) are often used and optimized for low-power usage and/or low-heat. x64 is often more optimized for maximum performance, at the expense of higher power usage and more heat. For many x64 CPUs you can drastically reduce the power usage if you underclock the CPU just a little bit (~10% slower), especially desktop CPUs but also laptops.
There are ARM and RISC-V CPUs that consume much less power, but they're also much slower and have a much more limited feature-set. You do need to compare like to like, and when you do the power usage differences are usually small to non-existent in modern CPUs. ARM today is no longer the ARM that Wilson et al. design 40 years ago.
And for something connected to mains, even doubling the efficiency and going from 7W to 3.5W doesn't really make all that much difference. It's just not a big impact on your energy bill or climate change.
There is no need to have something faster. The SATA 3 bus would saturate a 2.5 GB card anyway. The home network in Cat 6A so it could go up to 10 GB. We'll see what happens some years from now.
You could always use an rpi if you want to go with ARM, and you’ll want something with ARMv8.