Very timely, the final report has been released today.

I hadn't read the document you referenced, and I admit don't have the prior knowledge, nor the time, to fully understand all the implications of what it says. My opinion is then the result of reading and listening a variety of experts and news sources, and it will have some biases, for sure.

Still, I have skimmed the final report to see if there was something that I could understand from first hand (and to support my original point, not gonna lie), and I found this:

_The increasing penetration of variable renewable and distributed generation, further market integration, broader electrification, and evolving environmental and geopolitical risks place the European electricity system under increasingly challenging operational conditions, requiring higher levels of resilience._

Do you really think that my original point (as uniformed as it might be), namely, that the levels renewable energy currently present in the spanish grid require significative investments, was wrong?

Yes, I think it's wrong, or at least way over-exaggerated.

You can run a grid to supply approximately 80% renewables (long-term average) without significant technical changes.

Only if you want to get the last 20% to renewables, you get technical challenges, e.g. related to synchronization and load-matching. But that is also not unsolvable problems, e.g. instead of relying on the inertia of steam turbines you can "just" build specific-purpose fly-wheels to do the same thing. It's just less elegant.

Source: Volker Quaschning "Understanding Renewable Energy Systems", too lazy right now to look up the exact page.

This is also consistent with the section they quoted. Generally, the load matching in grids is done by the system itself. If you add more wind and solar, which depends on the weather and location, you have to more large-scale intervention, e.g. allow generation re-dispatch. But that doesn't immediately imply that this is a dangerous process.

I have not read the report yet, but in another thread someone gave a very plausible explanation of what happened.

The high levels of renewable energy happened to contribute to this incident, but not because of something inherent in renewable energy. All renewable energy sources are connected to the grid through inverters, and in Spain most of these inverters do not use an adequate control policy, i.e. they do not compensate the phase fluctuations of the grid, like the synchronous electromechanical generators do (i.e. they do not generate an appropriate amount of reactive power for compensation).

Technically it is easy to implement such control policies in all solid-state inverters, but it was not done in Spain because there were no incentives, i.e. there were no regulations specifying how the inverters connected to the grid should behave, otherwise than disconnecting when the frequency went outside a permissible range.