Well, if your goal is to get away from modern macOS, your options are Intel or PowerPC...

PowerPC you have Leopard or Tiger, the latter being "faster" but more limited. You can also install the version of Leopard someone added Snow Leopard beta parts to, which I do have on my PPC MacMini, but it still gives you limited returns.

With PowerPC you might be better looking at a third party. I hear good things about MorphOS, though never tried it. You could do Linux too.