> I think even the development system wasn't free either.

Metrowerks CodeWarrior was the original development system for PalmOS and was indeed not free (in either sense).

However a bunch of enthusiasts cobbled together some free development tools: the main parts were adapting the GCC and binutils m68k targets to PalmOS's constrained PIC runtime environment (it was constrained even by m68k standards); a tool to convert the resulting COFF or ELF executable to PalmOS's .prc database format; and a text-based resource compiler for generating UI elements using its own home-grown description language to express what CodeWarrior users were using a graphical UI editor to make.

That mostly still exists as it was back in the PalmOS days at <https://prc-tools.sourceforge.net>. And if you hunt around on GitHub you'll find a few people who've kept the code compiling with stricter more modern compilers.

(And see also <https://pilrc.sourceforge.net> for the resource compiler.)