you can include helldivers 2 since sony owns the IP but then you will have to include the SOTC Remake, Detroit, Until Dawn, Bloodborne, The Order, etc. BTW, Spiderman only happened because Marvel approached sony for Spiderman game and Sony contracted Insomniac to do it. It is as much a sony first party game as ratchet or killzone was before sony bought out GG.
Sony doesnt own the Stellar Blade IP. They published it like they published Nioh last gen. So yeah, HD2 counts. Stellar Blade 2 doesnt. There is no conspiracy theory here. We have actual budget allocations from sony showing a reduction in SP investment. And a massive increase in GaaS investment which did not pan out as they've cancelled literally 9 of those games. Jimbo had promised 12 gaas games by 2025. When Marathon comes out, they will have delivered a total of 3 with Concord literally being cancelled after launch.
You can include everything, lol. This is the right approach. For Sony, Death Stranding 2 is a first-party game, even them not owning the IP. So the same logic can be used for Rise of the Ronin and Helldivers 2 at least. Sony was involved with the development of these games since the beggining. And well, I'm sure Stellar Blade would be a completely different game without Sony involvement, or Lost Soul Aside may never see the light of the day if Sony had not supported since the beginning. So yeah, they have invested in these games, their support studio (xDev) worked on these games, so I really can't see the pratical difference between any of these games or any other made by a Sony studio itself.
I see the things this way:
2013: Killzone ShadowFall, Knack (2)
2014: Infamous SS, Driveclub (2)
2015: Until Dawn, The Order, Bloodborne (3)
2016: Ratchet & Clank, Uncharted 4, The Last Guardian (3)
2017: Gravity Rush 2, HZD, Uncharted Lost Legacy, Knack 2, Gran Turismo Sport (5)
2018: God of War, Shadow of the Colossus, Spider-Man (3)
Total: 18
2020: Demon's Souls, Miles Morales, Sackyboy (3)
2021: Returnal, Ratchet & Clank Rift Apart (2)
2022: HFW, God of War Ragnarok, GT7 (3)
2023: Spider-Man (1)
2024: Helldivers 2, Rise of the Ronin, Stellar Blade, Astro Bot (4)
2025: Lost Soul Aside, Ghost of Yotei, Death Stranding 2, Marathon (4)
Total: 17