Does the Breath of the Wild’s DLC add content before beating the main story?

You can purchase the DLC and it will not affect your game play at first; you will just have extra side quests in your logs.

While you pay for the extra content, the items unlocked from expansion pass are not automatically given to you. Aside from Master Mode, everything is side quest driven and you must complete the quest in order to obtain it. If you ignore these quests, it would be as if you never installed the DLC. If you do obtain these items, it does affect the game play in the aspect of what these items allow you to do.

This is true for the new story content that was released within the 2nd DLC. This new story is completely independent from the base game and you have the option to do the quest or not.

Yes. With the first DLC you can do the Master Trials once you get the Master Sword. Both of the DLC's add so many chests around the places not far from the Great plateau. As soon as you get off of the Great Plateau there is so many treasure chest that you can find. But some of them are far from the Great Plateau. But you don't have to beat the game to get a lot of OP armor. And I'd say that after you beat the game normally you should do the rest of the DLC.

Also, you can play on a Master Mode version of your save file making the game a lot harder.

I'd say that after you beat the Great Plateau head to the Coliseum of Death and get some nice loot from chests in the ground there. Oh ya, all of the DLC chests are hidden in the ground. To find them after you beat your first shrine outside of the Great Plateau you will get quests that will tell you where to find books that will give you clues about where to find them.

I think that you should get Majora's Mask as soon as you can from one of the chests, as it makes all monsters think that you are one of them. I used that so much during my play through of the game.

So yes, you can do lots of DLC stuff without beating the game. As a matter of fact, all of it can be done without beating the game! That is since for the Champion's Ballet DLC after you beat all four Devin Beasts, not beat Ganon, you can do the Champion's Ballet.