I can only speak for Deus ex: mankind divided since I have played that myself.

It is technically a sequel in the sense that there are other Deus ex games that came before it with the original Deus ex, invisible war and human revolution. That said every single one of those games has it's own self contained story with only some overlap between them. Mankind divided features the same main character to play from the previous game but everything else is different. From the location to the characters barring one which isn't an issue for reasons... And is also otherwise completely isolated from the first two games maybe with one Hommage that you might miss If you don't know the first game but then it's just a great character instead of the thing it's supposed to be.

Long story short, IMHO you can just pick that one up and play it without worrying about missing a ton.

Disco Elysium isn't part of a series (and probably won't ever be, super disappointing news there). Going in blind for that one is a good idea.

I don't use the subscription services much but I did try a month of Xbox Game Pass a few months ago. Ended up being disappointed by Persona 5 Tactica and a little bit surprised by Dune: Spice Wars. The plot pacing felt undercooked in P5T and Spice Wars reminded me of a bit of the early 2000's vibe for Dune games, probably a plus for some people. Ultimately it didn't keep me interested for more than a few hours (4X games have a very high bar to clear for me). Also tried Hi-Fi Rush, but didn't stick with it long even though it felt like a solid product.

Didn't continue Game Pass because I'm kind of a mercurial player, bouncing from game to game. What I end up sticking with depends a lot on vibes and mood. The service wasn't the best fit for me.

It's not a really big thing, but it is a pet peeve of mine (and some others); the name of the series isn't "Dues Ex" but "Deus Ex" (day-us ex), as in "deus ex machina" (day-us ex mack-in-a).

"Deus ex machina" literally translates as "God from (the) machine", and originally referred to a type of stage prop used in ancient plays, then in more modern times the term came to refer more generally to the sort of plot device that used that prop, which is a previously unmentioned person or thing that suddenly appears to save the heroes from an otherwise inescapable threat. At some time in the 60s or 70s it started to become popular to use it in a more literal sense in sci-fi stories about machine intelligence or cyborgs.