Competitive Pokémon is something I’d definitely recommend. It’s similar to speed chess in that there’s a game timer but you have like 30 seconds per move to lock in. VGC is an incredibly deep and constantly evolving meta game. The NAIC tournament finished yesterday and is on YouTube if you want to see it being played at a top level with great commentary.
You can practice singles and doubles for free on pokemonshowdown.com. IRL contests held by The Pokémon Company are always doubles, played on Switch and will be played with Scarlet and Violet for the foreseeable future.
I found the dark world very tedious, from what I remember. One aesthetic throughout the whole game that was very dreary to look at. Has been many years since I played it though.