Outer Wilds is my favorite game, it's just so tight between the narrative and the gameplay, there's not a second wasted and everything ties together appropriately and it doesn't overstay it's welcome so long as you're moderately competent at the sleuthing. Better yet is the DLC they released was equally satisfying while remaining a standalone narrative with it's own themes and mechanics, while still managing to tie in nicely with the base game narrative. I would suggest holding off on playing it until you've completed the OG, but theoretically you could do it anytime during the playthrough.

I guess the only downside I can think of is re-playability, being a mystery/puzzle game once you've acquired the prerequisite knowledge it's a bell that cannot be un-rung and experienced again with the same novelty. Maybe someday I'll go back, but until then I'll suffice with the tear-jerking OST of both game and DLC, as I'm reminded of the most humanistic and existential game I've ever played.