Hi! Congratulations on your onyx necklace!
I am a matchy matchy person, so for me, I would get a matching bracelet for sure, no doubt in my mind. It creates a beautiful set.
I dress super casual, and I feel matching sets help me look put together, even when dressed like a slob.

That said, there are lots of people who prefer not to match and like to toss on different colors, so this is 100% personal preference.
I love love love blue agate, so if you do not need to match, and want blue agate, I would get it.
My thoughts on non-matching stones to share...
I personally think MOP works better with onyx, they complement and do not compete. MOP also glows on the skin; it's such a gorgeous stone. If you want a color though, MOP can be boring as it's not a "color."
My second choice to complement onyx would be carnelian. Both are bold and stand on their own, yet they are strong enough to work with each other without one getting lost.
My last choice is actually blue agate with onyx. Onyx is a stark intense black, but I see blue agate as a happy bright blue. I am not fond of the intense black and bright blue combination together. It may not compete that much because the bracelet and pendant are far away, but even so, I personally do not like this color combo. For me, the lightness of the blue agate gets lost a bit next to the heaviness of black onyx. The intensity of the black onyx then is also softened next to the bright happy blue, so the look together for me is neither bold black nor happy bright blue. The blue agate to me is a happy summer color and should not be weighed down with black. That's just me though, and again, the two pieces are far away, so it might not bother you or you might even like it.
I just want to share my thoughts for another point of view.
As I said earlier, if you don't mind not matching and you don't feel a off when you wear it, go with what you love.
I think once you try on a few bracelets, your gut will tell you if it works.
I hope you try the matching on the onyx too. I personally think your pendant and a matching bracelet would be perfect. It's the perfect amount of that harsh black to make a statement, but not too much black to make it look somber or too goth. Plus, you can wear the oynx set all the time with your outfits. Blue agate to me does not work with everything and for me, definitely not in the winter.
In the end, it's 100% preference. There is no right or wrong, but I think it's always good to hear different points of view so you can decide what works best for you.
Hope this was helpful. Good luck to you!