Google pixels from pixel 2 to pixel 5a can upload to Google Photos in Storage saver Quality at no charge, i.e they can backup unlimited number of photos and videos without consuming from your Account Storage . Google Official Anouncement
Pixel 1 can upload in original quality i.e without compression. but there are no pixel 1 in this deal.
Google Pixel vs Google One Subscription
At this price, Assuming that it takes ≈5 years of Google One subscription to recover the cost of the Pixel phone purchased today, i.e. the break-even period is ≈5 years, one must remember that the hundreds of GBs of pictures and videos over this period will continue to be stored for free for life, if backed up using a Pixel device, but one has to continue paying a fat subscription for these files for the rest of life with a Google One subscription. xdaforums Discussion
There is lot of discussion online how you can automate to Sync media from your other phones to pixel then backing them to google photos.
Quote from XDA:
"I'm told Pixel Experience ROM (and some other custom ROMs and mods) allow for spoofing the device details to make Google Photos believe that the device is a Pixel, thereby unlocking unlimited storage on Google Photos."
Sounds interesting. I have a Pixel 1 128gb with a bulging battery so might need to look at the ROM approach for a more reliable phone in the future. It's my second Pixel as the first one had a bulging battery as well and was replaced well outside of warranty by Google free of charge. You just need to harp on about safety and ACL rights to get a free replacement refurbished phone.