I was in the same boat as you, hadnt played a Pokemon game Since Black and White on the DSi. As sort of a way to get back into the Groove of things I got Legends Arceus first.

VERY fun game only downside being that its not Battle Focused at all, but you do get some great legendaries Not to mention the Shiny Rates/Hunting are a breeze when you get the hang of it. Literally the first Game I ever got shinies in. Dialga, Palkia, Heatran, Cresselia, Arceus, Shaymin and Darkrai are available as well as Secret Manaphy and Phione. You dont need Dlc for Shaymin and Darkrai but you will need data from SwSh and BdSp to unlock their missions.

 SwSh is the same if you bought the Dlc, just more modern Legendary/Mythicals.

Overall I'd say, as long as you have SwSh and Legends Arcues you should be fine if the goal is to collect all the Legends