LEO is just for demonstration

They can! Continental drift measurements have been done using radio interfometry since at least the 1980s: https://www.nytimes.com/1983/07/05/science/science-using-new... (Though that 1983 article does suggest they were still working on the accuracy). Certainly radio interferometers can measure displacement of dishes after earthquakes.

For my second year degree project we did a project on space based VLBI, being just theory we were looking at a bird cage orbit, as you go out further one needs to fill out the “UV plane” (I’m just recalling bits on my 6 week study on interferometry).

ISM is a thing for continental VLBI, as pointed out by the head of Jodrell Bank when I went to a public out-reach talk in London.

Whilst not interferometry it would (might) be good to have a Gaia type galaxy mapping mission further out, although it if there were a few satellites they could account for the larger orbits.

I’m not sure if a birdcage orbit would improve things much the cost of having a high inclination orbit out of the solar plane would be huge.

On the ground? Sure. Orbiting? I'm amazed they reach sub-mm. The gravitational field of a planet or star is not uniform, subtle variations in their density are enough to impart tiny variations to an orbiting object, and they add up over time.

Can't find a reference right now but I recall someone proposed even a positioning system relying only on accurate gravimetric sensors and a (very good) map of the strength at every relevant location on Earth. Good for submarines, GPS reception is not so good underwater.