I don't think Sony's first-party games would land on the service Day 1. At least not initially. Sony's business model almost entirely depends on game sales since most of them are single player experiences, and with their gaming division making the most money for the company, Sony wouldn't want to lose too much revenue.
I'd think they'll launch the service with classic titles, hopefully their way of tapping into their PS1/PS2 catalogue, and some PS4 titles like God of War (2018), Spider-Man, Infamous: Second Son, Days Gone, The Last of Us Part 1 and 2, Bloodborne, etc. then start to add PS5 titles something like 6 months after a games launch to get as much from initial sales as possible. Once Sony has a number of subscribers they're happy with, then they'll start to add PS5 titles Day 1.
Hopefully I'm wrong though :) maybe add a PlayStation PC launcher as well and connect the services!