I'm about 75% sure that the RAM will simply run at the maximum speed that the motherboard is able to. The extra headroom may be "wasted" but for $12, it seems like a reasonable choice. The CAS latency of 16 isn't the "best" (CL14 is a little better in the DDR4 zone) - you might have been able to get better CL with slower RAM. But overall it probably will not make much difference.
Your motherboard would support "onboard graphics" if (and only if) you bought an APU instead of a CPU. In your case, you're plugging in a dedicated graphics card, and the onboard graphics will be 100% unused (and unusable.)
Overall I think you made very good choices. I'm sure I could "nitpick" but it would just come from a place of bias... (I prefer ASRock/Asus and Gigabyte motherboards, Samsung SSDs, Corsair power supplies, etc... but in computer tech terms, I'm an ancient dinosaur.)