I recently put Alpine with i3 on a Raspberry Pi 4 Model B and I'm super impressed with how snappy it is. I find it much better even than Raspberry Pi OS Lite.

Yep, Alpine works well. A GUI can be tricky, though. And none of the RasPi tools (e.g. `raspi-config`) will run because of the different libc.

So, running it on a Pi 5 CM in an IO board, there's no way to tell the Pi what device to boot from.

Same here, I put it on two very old RPi 1 and was amazed at how low the footprint is. I wish there were images available for other SBCs as well, mostly Allwinner based ones (OrangePi, NanoPi, etc); probably I did something wrong but building them from scratch turned out more complicated than expected.

I can vouch for it, I had an RPi that was translating a serial port to TCP/IP in a difficult to access location, and it stayed doing its duty for years, Alpine is very solid.

Important to mention: Alpine can run from RAM like TinyCore