We can get pretty far along! From magnetic system design, vacuum vessel, RF heating system, cryogenic system, tritium fueling, etc we can start making a ton of progress today. The main things we still need to learn that can influence the design is advanced divertor scenarios and what are best material choices for plasma facing components (PFC's).

How certain is it that a tokamak is even able to be run in a stable manner? What if it turns out that a stellarator would be better? Or is that already validated by now?

The stellarator design makes more sense to me as well and speaking of it, those guys will build a stellarator on land:

https://www.proximafusion.com/press-news/proxima-fusion-and-...

Problems are still many, though (Paper:)

https://www.sciencedirect.com/science/article/pii/S092037962...

Saw that this morning as well, it's awesome. I do think long term especially for grid applications stellarators could be great... note that their design is ~2.7GW I believe. We're talking gigantic traditional power plant size, which the world needs, but the first generation of these facilities will cost multi billions of dollars. Also with the stellarator, doing non planar HTS coils and the associated manufacturing challenge is a very very hard problem.

Well, definitely good luck with your approach as well!

I guess the biggest hurdle will be stable operation, without having to replace too many broken parts too often?

Tokamak requires regular shutdown as far as I understood and that is quite a lot of heat stress for all the parts I believe, along with the radiation etc.

(But I lack the background to really debate on the pro vs cons of tokamak vs stellarator, I just have opinions here)