Google Photos is a solid choice note it only catches exact duplicates and the free space is limited across the account, so consider one shared family account so everything lands in a single library. Before uploading, do a quick pass on a computer to de-dup and sort files into simple Year/Month folders; that keeps order even if dates are messy and makes the offline copy straightforward. For the offline backup, mirror that folder tree to an external drive (two if possible) with a basic sync tool, then leave auto‑upload on for the phones going forward.
