I actually like the idea of actual tournaments for Leagues, similar to Sword and Shield.My advice would be to not limit yourself to the 6 slots.
These EXP Share forced games are fully designed around the idea that you won't stick with the same 6 pokémon throughout the entire thing, but instead will use everything you like, so having a bigger team of rotating pokémon is pretty much a necessity if you don't want to get seriously overleveled (or having to constantly worry about micromanaging your team to avoid that). I kept a team of 6 in Sword and I had to micromanage so much that it ruined the immersion.
I fully intend to have a big team this time. My only problem is the fact you can't rematch the League, and it really doesn't sit well with me the idea that not every member of my team will be a "Champion".