I'd (and I did) manually update to the September 2018 factory ROM to start. One step. (You'll have to unlock the bootloader but, if you're not going to run Magisk [root] or TWRP, you can relock it as soon as the ROM finishes installing.)
As for the system cache, if the phone isn't stopped by removing the battery, and all the system software running in the phone is written correctly, the cache gets cleared when you shut the phone off. (It's also one of the fastest shutoffs I've seen.) The only reason for a cache wipe is an app that doesn't clean up after itself or shutting the phone down before shutting the app down. (A properly developed app should clear its cache as it closes.)
So Google does the hardware, Google does the software and it all works out well. (As far as the a/b system, if anyone has the patience to check through my years of posts, you'll see that I asked why they weren't doing that years ago. It's 1998 technology [at least - I worked on developing an update system like that in 1998] and it sure saves you from things like the battery dying during an update. The phone just boots to the unupdated side again and you get to try to update the other side again. Finally. [Can I ask for royalties?])