I would use Americium-241 instead, longer half life and much more availability.

Lower power, but a telescope like this does not need constant power, so some kind of short term power storage (capacitor I would assume, or some kind of ultra long life battery) could handle that.

in terms of the potential plutonium shortage, wikipedia: Americium-241 is not synthesized directly from uranium – the most common reactor material – but from the plutonium isotope 239. The latter needs to be produced first

Last I remember, RTG manufacturing was very constrained, period. And that was before Russia took a massive shit in Ukraine and got themselves embargo'd by most of the rest of the developed world.