I think Sekiro is the better game of the two, but since you're one of the lucky few who managed to get a PS5, you might as well play the game that defined its launch window for me (Demon's Souls). It's a bit easier than Sekiro, and Sekiro is like 50% souls-like and 50% its own thing, so that might not be what you're looking for.
Also, you don't HAVE to play all three Souls games. I highly recommend you skip 2/Scholar of the First Sin entirely. I treat Demon's Souls, Dark Souls 1, and Dark Souls III as a triligy since they were all made by the same team with the same director and thus they have a similar feel (even if they're very different in design). Dark Souls II was made by a different team and it feels like a fan-game or something, not a sequel to 1. If you are going to do Dark Souls, play Demon's Souls, then Dark Souls Remastered, then Dark Souls III. Or just skip Dark Souls Remastered because it has aged ungracefully.
My ranking for all the FromSoft Soulslike games: Bloodborne > Dark Souls III > Sekiro: Shadows Die Twice > Dark Souls Remastered > Demon's Souls Remastered >>>>>>>>>>>~ Dark Souls II.
the Nioh games (Also enhanced and exclusive to PS4/5) are also quite good. Samurai Souls. They're closer to Souls than Sekiro is, but with a single level and area design more like Demon's Souls as opposed to the semi-open-world of Dark Souls and Bloodborne.
Wow, Demon’s Souls is your second least favorite? That makes me feel less excited to play it over Sekiro. :/
I think overall the atmosphere and enemy design in Demon’s Souls are more appealing to me than Sekiro (tho I do still like Sekiro’s aesthetic and art style), but I do worry that DS is so much older than Sekiro and thus might feel outdated/archaic. I’m not sure which combat style I’ll like more, but I liked Bloodborne’s combat, and it seems like Sekiro’s combat more closely resembles that of Bloodborne.
Thats an interesting concept to potentially skip DS 1 and 2. Would I not be missing anything important or vital, story or otherwise?
Don't let this discourage you. every game in this list aside from Dark Souls II is in my top 25 of all time! They are all great, it's just sorta clear that Demon's Souls was a fantastic prototype for the better, more refined Dark Souls 1 and 3.
My Console Library:
PS4, PS3, PS2, PS1, WiiU, Wii, GCN, N64 SNES, XBO, 360
3DS, DS, GBA, Vita, PSP, Android
Top 6 this generation:
Bloodborne, Sekiro: Shadows Die Twice, God of War, The Legend of Zelda: Breath of the Wild, Dark Souls III, Red Dead Redemption II, Rock Band 4