Install PhotoCocoon on the new phone. Sign in with the same email you used in the app. Choose restore to the library. Stay on Wi‑Fi, cool, charging. A large library takes nights. That is the originals being large on purpose.
Before you sell the old one
Confirm the widget said all backed up. Sign out. Erase in Settings. The attic does not live on the handset you sold.
iCloud
You can still use iCloud Photos. We are the independent copy. Do not turn iCloud Photos off until originals are on a device you hold. Keep originals.
Related: Restore · Phone died
$12 / year for 100 GB. $20 / year for 1 TB. Paid in the iPhone app.