That makes sense, as the Pi is as easy as x86 at this point. I almost never have to compile from scratch.

I'm not a compiler expert... But it seems each ARM64 board needs its own custom kernel support, but once that is done, it can support anything compiled to ARM64 as a general target? Or will we still need to have separate builds for RPi, for this board, etc?

Little bit of both. Pi still uses a sort of unique boot sequence due to it’s heritage. Most devices will have the CPU load the bootloader and then have the OS bring up the GPU. Pi sort of inverts this, having the GPU leading the charge with the CPU held at reset until after the GPU has finished it’s boot sequence.

Once you get into the CPU though the Aarch64 registers become more standardized. You still have drivers and such to worry about and differing memory offsets for the peripherals - but since you have the kernel running it’s easier to kind of poke around until you find it. Pi 5 added someone complexity to this with the RP1 South Bridge which adds another layer of abstraction.

Hopefully that all makes sense. Basically the Pi itself is backwards while everything else should conform. It’s not Arm specific, but how the Pi does things.

Apart from very rare cases, this will run any linux-arm64 binary.

Fot the Pi you have to rely on the manufacturer's image too. It does not run a vanilla arm64 distro