I have the feeling it will be more economical to just use a low-temperature superconductor and cool it down with a closed-cycle cryocooler, rather than try to get a large enough pressure vessel that can maintain such high pressures.

A few years ago, that was the thinking when a 'high temperature' (much colder than room temperature) superconducting cable was trialed in Germany [1].

[1]: https://www.tdworld.com/overhead-distribution/ampacity-proje...

[2 with follow-up presentation]: https://snf.ieeecsc.org/abstracts/stp475-ampacity-project-%E...

With current technology, definitely.

You want high temperature superconductors because they can stand larger magnetic fields. So, it is certainly easier to keep them cool than pressurized, but the cold ones are much less useful.

Wrap them in carbon nanotubes :)

My immediate thought was to wrap them in some sort of diamondoid shell through some sort of mechanosynthesis process, but maybe a carbon nanotube would be stronger?

couple it with some double walled carbon nanotubes and you can get upto 700 GPa

https://www.nextbigfuture.com/2019/01/fabrication-of-ultrast...

I wonder if one could just stuff the tubes with hydrogen, then lanthanum powder, then just let them expand? They might auto-heat from the pressure, and assemble themselves, if you just unfroze them.