The Mario + Rabbids: Kingdom Battle DLC, Donkey Kong Adventure was basically too small to be a sequel but too big to be a basic add-on to the game. As such, the developers made it so that, once you load up the game, you choose from either the main game or DKA, and then it shows you either your 3 save files for the main game, or your 3 save files for DKA, depending on which you select.

While any% is the only category currently on speedrun.com for MRKB, there have been extensive talks on the Discord for future categories like 100% and the game's form of NG+ (All Levels WM). All these categories have equivalent forms in the DKA DLC.

Based on this, do you think it's worth it submit the DKA DLC as a separate game in the same series, or just add all these categories to the existing leaderboard?

IMO if it's seperate from the main game, make it a new page. Xenoblade Chronicles 2 - Torna: The Golden Country is in that boat, where it's DLC, but basically acts as its own game.

the Expansion & DLC game-tags are there for a reason.

How similar is it to Shovel Knight getting campaigns added?

You made me think of two cases I know where a DLC was later released as a standalone game -- Assassin's Creed Freedom Cry, and The Last of Us Left Behind.

Sadly there's no helpful precedent here, as AC folds Freedom Cry into the Black Flag board and TLOU has a separate board for Left Behind.

To me, separate seems right.