So I follow a number of China scholars and experts and I've yet to see any consensus about what these military purges actually mean.
It could be about corruption. You see this in the Russian military where paid-for tanks didn't exist because the generals had pocketed the money. It could be to have an expansionist policy. It could well be to not have an expansionist policy. The point is that nobody really knows yet.
But the string I really wanted to pull at was this idea that China isn't a "rational actor". It's lazy and really a thought-terminating cliche. It's certainly no basis for analysis or policy-making. It's kind of the final boss of justification. "Putin/Saddam/Xi/Castro/Maduro is crazy". That really just means you don't understand what's going on or want to ignore the facts.
We now have 50+ years (since really the end of the Cultural Revolution) of China acting in a very rational, very intentional and very long-term way. Xi's own history here is pretty interesting. He went from privileged child (his father was one of Mao's lieutenants) to being banished to working his way up through the party's ranks over decades.
It's a mistake (IMHO) to view Xi as a singular actor, let alone as a irrational autocrat. While the PRC and the CCP might be relatively new the systems and political structures can probably be traced back thousands of years. I'm thinking particularly of the bureaucratic reforms of the Qin Dynasty some ~2300 years ago.
What cannot be ignored is that a billion Chinese have seen a massive improvement in their living conditions during their lifetimes. Almost all of the people pulled out of extreme poverty in the 20th century were because of China (~800M). So although China is authoritarian, the government is extremely popular because of that increase in living conditions. It's something that we in the West have a hard time fathoming because our living conditions have been in decline since at least the 1970s.