I am in the same boat, so I am eager to hear your answers. I have 3 who are brilliant and I have yet to find something that is challenging for them. They are on their own all math block because everything we do is way too easy for them.
I also use teachers pay teachers resources. What are some of your favorites? I think one idea I might do is have them make videos of themselves teaching concepts, possibly for lower grades? My kids also do ixl math at the 7th grade level.
I've used a few activities from this site:
My other problem is how fast these kids can work! I plan these long activities for them and they finish them in a few days and need something else!