I'd wager it's a spring 2021 title.
Miles Morales and Demon's Souls are probably the launch headliners (and Astro Bot). If there's another, I'd bet on Sackboy.
Yeah I can't see Ratchet launching with the PS5, especially if Sackboy is a launch title. You don't release 3 3D platformers all at the same time -- you'd cannibalize your sales that way.