I have an original Pi 2012 running buildroot. It's a perfect fit. Need just enough to run a Linux kernel and ser2net for doing RS485 stuff with solar inverters.

I have an original Pi 2012 running buildroot. It's a perfect fit. Need just enough to run a Linux kernel and ser2net for doing RS485 stuff with solar inverters. I think the image size was around 100MB and no volatile filesystem whatsoever. Buildroot was surprisingly easy to use. Use a menuconfig to pick what you need and a burnable image for your SD card comes out the other side. Think I only spent an hour on the whole project.