Yeah his site isn't currently the clearest for that.

Here's a direct link to his build instructions [1] and I've made a temporary by-item BOM [2] while he improves things.

PETG is great, ASA and ABS might be better if you have the capacity.

     [1] https://la3emedim.fr/assets/SMALLEST%20-%20README.pdf  
     [2] https://docs.google.com/spreadsheets/d/1XXZB4jRtvtlDIqlR69C0FRri8x8lZDpxO6ZGnSp3L2Y/view

What about glass or carbon fiber filled PA6?

All plastic is going to warp over time. So that probably seems like overkill unless you're planning on abusing it.

That said, I would give PETG-CF a try. I've made parts with that and it's certainly been good, and is definitely better than PETG.

I don't have enough experience with nylon to assess whether it would be suitable for a full telescope, sorry.

Pla-cf is generally more brittle because of poor layer adhesion due to the fact that carbon fibre doesn't stick to carbon fibre. Pla also doesn't need the additional stiffness CF provides so it's just a gimmicky filament.

Petg is a better option.

PLA-CF isn't a high strength filament, but I do really like it for large functional prints that need to be stiff and look good. It has a nice texture right off the printer.