Overall you did a good job for the first time cooking that cut.
From looking at your earlier pictures, the way the butcher cut your brisket, the thin part was what was left of the flat. It looks like you didn't separate the two muscles before you slicing it. The thicker part of the piece of brisket you have is the point. I can tell this by the marbling and the shape. Where the flat and the point connect, those two pieces the grain goes in different directions. In your first picture of slices, you were cutting across the grain, which makes a more tender slice. In your second picture when you came to the thicker part where both muscles meet, I can see when sliced part of the slice is with the grain, and part is against the grain. Slicing with the grain make makes it tougher to chew. Also I like the keep my slices less than 1/4" thick, unless I overcook it and I have to slice it thicker for it to keep from falling apart.
You should go by temperature, and tenderness; because different cuts may cook differently. When a fork is able to be inserted easily, your brisket is done. Many like to use the temperature probe, and use that to test tenderness. When you are able to slide the probe in and out easily, that is another sign that it is done. For myself, I don't like fall apart brisket, I like a little "give" to it.