The “regulated capitalism” you think of came as a direct result of a threat of socialist revolution, brought by interventionist USSR. Without a threat like that, capitalism degrades rather quickly into what you see today and way beyond. Capitalism itself is an awful system based on greed and exploitation, and it will blow through any guardrails you try to put on it given enough time.

I really worry about fully planned economies being really fragile.

As opposed to the very stable free market economy of the US, which is about to go through a third “once-in-a-lifetime” recession in 20 years?

Meanwhile e.g. Chinese economy is mostly managed by actual economists who aim to balance economic development, poverty alleviation, and quality-of-life improvements. The impact of their crises are felt most by those on top, not only financially but also in terms of prison sentences if they are to blame.

Pure socialism has been proven to be just as historically toxic as pure capitalism.

This is precisely what capitalists would want you to think. It is indeed what I thought a few years ago before someone pushed me to read a bit of Marx, Lenin, and Kropotkin, and look at the history of rising socialist states critically. If you actually examine what socialism has done for the working people, it becomes obvious that it is a flawed system but a massive improvement over capitalism.