You could add some races/challenges as Individual Levels. Just having DLC categories isn't gonna get anyone to speedrun this and I really want to speedrun this. I know it's a bunch of work to add all races/challenges but it'll make the community have way more fun.
1.) As there are 200+ individual challenges in the game it would be insanely difficult to add them all to speedrun.com along with a section for each platform, but we do have a google sheet that has most of the individual challenge records documented at leaderboard.steepcommunity.com
2.) there are no base game categories as its quite hard to define once the game is 'finished' so theres not one category that everyone can agree on as the definitive main category which is why we just have the dlc speedruns