To the question, "what parts would you put up"? The answer is, "another identical pistol".
Seriously, you don't know what is going to break (if anything) so your best insurance would be an identical pistol. Plus, it's probably cheaper in the long run to buy an entire replacement pistol than to assemble all of the parts that you might need.
I don't think a Glock model 20 is fragile. If treated with a little common sense, I think one could keep a G20 in operation for decades.
However, on the other hand, I'm not convinced the Glock 20 is going away. So before you start laying out cash for spare pistols, my advice is to sit in the tall weeds for a bit. Even IF Glock discontinued that model.......there are a lot of Glock model 20's in circulation. They aren't going to evaporate overnight. The opportunity to acquire a second Glock Model 20 will not instantly close if Glock discontinues that model.
I also don't see Glock USA making a pistol tailored for the California market. There's no money in that. California has a huge population but there are not enough "gun people" in California to offset the market in the other 49 states. Despite the huge overall population, the market is just not there.