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There's also Buildroot which has a bit of a learning curve (it's still easier than Yocto/Angstrom IMO) but is capable of producing highly customized, tiny embedded Linux images. At this time, there are board configs for pretty much every Rasperry Pi variant out there.

The functional equivalent of a Debian base install, including kernel, bootloader, and systemd init (approx. 350 MiB) is about 20 MiB (compressed) when built with Buildroot.

Build a static binary (with musl perhaps) and put it in an initramfs at /init. An example https://github.com/mkj/rpi-flipflop/blob/main/Makefile#L9

Have you tried ArchARM? It's pretty lean and fits into those old 512M SD Cards.

If you have a 512mb SD card, you should probably not use it anymore. Even if you don't need the storage.

I use a 64 mb CF card for photos

I haven't heard of that one (I did check out a version of puppy and tiny core). I'll check it out, thanks!