I can only speak for the first 3 paid dlc, since I still haven’t played the castlevania one. The first 3 essentially add an alternate route to the game, so about 6ish levels and 5 bosses between them. With the exception of the queen and the sea*, the DLC content isn’t that much harder than base game stuff, and it’s all accessible in the games easiest difficulty (generally they’re easier than stuff from the free updates lol). They’re pretty good at adding a bit of extra variety in what routes you can take through the game, and add some extra items. Compared to some other games I play (stellaris), the DLC do feel pretty optional which is nice to see, and to the best of my knowledge they were also partially intended to support the creation of the free content as well.
*the bosses in that dlc are rough ngl. Queen boss fight is really fun though.