That's actually pretty f@cking good for a first try!!!! To me it looks like the tiny gap is the width of the pencil mark. Perhaps scribing the second cuts using a knife or a chisel would get you to the precision you seem to want.

You can use sawdust and glue to make up some wood filler and seal that right up. Even the pros do it.

That's also very thick wood for a dovetail. I'm a little lazy with respect to woodwork, so I'd recommend a simpler joint that provides similar structure