Do you think China would allow the US to toe Korea’s red line? Would relying on the PLA for the conventional deterrent not be the better strategy?

Honestly i don't know, and i think neither do the Koreans. This isn't the 1950s and 60s anymore. I am not sure that anyone would intervene directly if the DPRK was attacked. They may give them weapons and send "advisors" like NATO is doing with Ukraine (and even then i think there is a chance that neither Russia nor China will do as much for the DPRK as NATO are doing for Ukraine), but the ones who will mainly have to do the fighting are the Koreans themselves.

You have to put yourself in their shoes: They lost their main ally once before when the USSR collapsed and that left them in a very bad position, they have learned that they can only really rely on themselves. Unless there is an explicit treaty with the PRC that guarantees that China will defend the DPRK, which i don't see China signing any time soon as they openly state they are opposed to the formation of blocs, the DPRK has to do whatever it deems necessary to ensure its own survival.

And as the Ukraine conflict has shown you can never have too much war materiel. In a large scale conflict you burn through an immense amount of munitions and equipment. So the larger your stockpile and your military industrial capabilities the better. How much exactly they should invest in military vs civilian economy that is up to the DPRK leaders to decide, we shouldn't presume that we know better than they do what the material conditions of the DPRK allow and call for. These are hard decisions that i don't envy them having to make.