Pairing the cpu with a 5070ti.

I game, stream, make yotube videos in davinci as a hobby.

Is the 9950x3d a waste of $200 or will I notice the cores?

Depends how heavy you want to go on creative content in the future
A lot of creative applications don't benefit from the V-cache so something like R9 9900X would be better for a creative

BUT

Modern Cpus are overkill for a home user now. You won't be disappointed with anything that's high end

I own a 9800x3D and i switched from a 13900K. It was damaged tho so it had to be replaced. But I deff notice a huge difference switching from a 24 core cpu to a 8 core cpu. I am gonna buy a 9950x3D in the future or the next AM5 high core X3D chip if it launches.