What Happens to My Photos If I Cancel Google One?
They are not deleted immediately, but you cannot add more, and after long enough over quota Google may remove them.
Cancelling drops you to the free 15 GB. If you are over that, you cannot upload anything new, and Gmail may stop receiving mail. Existing photos stay for now.
Google's policy allows deleting content from accounts that stay over quota for two years. Whether that is enforced varies, and it is not a thing to gamble a family archive on.
Export with Takeout first and keep your own copy. Takeout JSON Metadata Fixer makes that copy usable rather than a folder of undated files. It runs on your own computer and is free for your first 500 photos.