I woke up one day thinking I had that problem with my Pixel 3. Literally all the things described were going on and I was about to throw in the towel and rage post to reddit, but then I had a brief showerthought that maybe the battery was bad and the charging firmware was being retarded. Pixel 3 had this habit of autobooting when charging from dead when it thinks the battery has made it far enough to survive boot... it boots into this "battery charging" mode and then it will boot up when that's gotten far enough. But when its wrong (because the battery is dying) it never has enough power to make it past some initialization stage. Ultimately I "fixed" it by draining the battery completely (until the phone would not respond to anything) and then plugging it in to charge and watching it like a hawk to jump in and long-press power to force shutdown immediately when it tried to boot to keep it in powered-off charging (not the mini-boot it does where it shows you battery charge). It happened a second time and after rescuing it again, I replaced the battery and all was good.