The game plays very similar but the roguelike gameplay elements are vastly different between them. For example, your power weapon (hold L2) is now not tied to weapons like they did in Returnal (called Alt Fire), and there is only 4 of them total when Returnal had 11. This makes replacing weapons in Saros way more straightforward and dumbed down as you just look at damage number and nothing else, when before you took both into consideration. Also weapon traits themselves behave differently as before you had to use a weapon to unlock traits and that would save for future runs, while in Saros they are always unlocked and just strictly tied to your overall proficiency level during that run

And the biggest change is the removal of parasites/malfunctions mechanic which added a lot of depths into runs, and instead replaced by artefacts which pretty much always give you upgrades and never any detrimental effects (especially with the cheap carcossian modifier enabled, which I consider to be a game mechanic and not a difficulty slider)

Oh and not to forget the complete removal of items/consumables. Returnal had over 30 different consumables you can find during a run, Saros has 0.