[blucaz]

> So how do I replace any of them with my own, small tool?

You write them with an equivalent public interface.

> They have no stable interfaces.

This is nonsense. All the D-Bus (and now Varlink too) interfaces are stable, documented and we do not break backward compatibility.

> yet nobody was able to write a replacement.

That's because actually doing things is hard, but complaining on social media is extremely easy.