If you want to, you can have that; just take postmarketOSs new stable release when it's out with your prefered mobile shell and put the bare minimum of apps you want/need on there and you'll be set.

As for KaiOS, I doubt we'll see that soon, as IIRC like FirefoxOS it's designed to run on Android kernels. With GloDroid being mostly dormant and incomplete, I doubt we can pull that off.