I’ve just reached level 50 playing a character that is mainly archer and has the Dawnguard crossbows, which are pretty powerful. I’m wondering if the Dragonborn add-on is worth doing at level 50 or if I should wait and create a new character before trying it. I like a challenge – but not too challenging.

Yes. It's not hard at that level but it is certainly isn't a walk in the park.

I'm a level 65 and I just started Dragonborn and I am enjoying it. I've actually died a few times from some of the powerful cultist (lightning everywhere). Although I play on Master difficulty. If your crossbow is as powerful as you say, you should have a good balance of some fun and a challenge.

Worth playing at 50 indeed and I believe most items you found in DB will be at there level cap by 50.

If you are on the PC i'd suggest getting a few mods to make things harder..But other than difficulty its pretty enjoyable even at level 81.