You can just get them and refund if you don't think they're challenging enough. Just make sure you read the conditions - I think that there's a time limit on DLC refunds.

It should be mentioned that the DLCs don't just add things for the player - the aliens also get buffs, in the form of new mutations like Vile Tongues, but also with things like Mutator structures.

The Planetary Campaign DLC is totally worth it IMO. On the toughest difficulties it really is a massive challenge, even to veterans of the game (although the RNG does come into play in terms of what techs are available and at what point). If the main campaign and regular skirmishes being too easy are a problem for you, then I'd recommend both DLCs without hesitation.

Whether or not you decide to try the DLCs though, I'd recommend the weekly challenges. As you're up against other players more than the alien AI, the difficulty is dramatically increased. There's a great sense of achievement from getting one of the top scores in any category, especially as some of these people have been playing the game for literally years.

The campaign is 100% worth it. You can set it to extremely difficult too. The DLC is well worth it.