For a while (pre-Tensor) Google seemed to have gotten away from the carrier being in the middle of the updates (at least for phone bough unlocked directly from Google). Somehow with the P6, Verizon has become a roadblock again. On my Nexus 5x, P2, and P4 the check for update button actually worked and I could get updates immediately.
Something changed with the 6. Not sure if it is the Tensor or Verizon. The 7s seemed to have gotten it, so it could be that the 6 is just plagued. Waiting for the 8 and the iPhone 15, but my 6's days are numbered.
Edited to add... The sideload was easy and quick. Just annoyed I had to do it in March and again in April.