My chain looks flabby when on the smaller sprocket.
Not sure it was like this before. You can see the picture.

From the second sprocket and up, the chain get straight.

I don't have a tool to check the chain wear.
But used the 12" trick, and have 305.5mm instead of the planned 304.8mm.

Should I remove one link? Or tighten something on the derailleur?

It looks like you need to remove a link. 

Did you follow the SRAM setup instructions when you first installed it or did you use it at the length it came?  I needed to remove 3 or 4 links if I recall correctly on my 256 with a 32T and X01 cassette.

After removing a link, it looks much better now. Thanks.

I followed SRAM instructions initially, at least I think.
As I'm fiddling so much on my bike. I'm surprised I didn't see it before.