That is an amazing first brisket result I am seeing in that pic right there. The way you described the tenderness tells me you were very close to perfect, maybe just a tad on the overdone side. Personally, that's about where I like to make them for myself at home.
The perfect brisket flat slice, sliced against the grain as thick as a pencil, will hold together when picked up, and when you grab both ends and pull it will pull apart easily with just a little bit of a "snap". If you feel the resistance, it is probably not quite done. It should snap apart with such little resistance that it really takes no effort, yet it holds itself together.
That is a very thin line of perfection. The slightest bit more done and the ends will fall of the slice. A little bit less done and the meat will not pull apart as easily. As close to this as you can get is what you are shooting for.
Honestly, from what you described, you were very close, if not actually there. My guess is when you sliced it, the ends tended to fall apart. But then I wasn't there so I really don't know.
Regardless, give yourself a HUGE pat on the back for making such a fantastic first brisket!