Is there any danger that a Google account could be blocked when I signed in with microG on my FP5. The Google security page told me, someone signed in with a Google Pixel 2. Well, that couls be me, disguised by microG. BUT, are there ANY Pixel 2 still running or is this highly suspicious and I should better be afraid of being blocked?
Google photos holds all the treasures of my past…. 
it’s just an opinion, I can’t know - but I don’t think there’s any discrimination as to the device info that is returned to Google, successful authentication is the only relevant criteria.
I think the issue with the device spoofing is the confusion itself it can cause, when users enable 2-step verification and wonder which device it is.
As to Google Photos, external drive + “Google Takeout” once to get them in one go? hedging risk. Loosing 5 years of photos in 5 years better than loosing a lifetime of photos.