Hi there and welcome!
Seems like you did pretty well for 2nd smoker ever and 1st time brisket :D
To solve the rock hard parts issue, just cook fat side down since your heat source is coming from under. The fat will protect the meat from getting rock hard.
As for trimming, I wouldn't worry about the silver skin that stuff will crust up and burn away.
I don't trim much fat unless it is clearly more than 1/2 thick then I just cut it down some. I honestly spend no more than 30 seconds trimming fat off the top of the brisket.
HOWEVER, I do trim out the deckle fat and sometimes on the opposite side of the deckle, there is a big seam of fat that I cut a good chunk out of. That's it for all my fat trimming!
Now I DO trim away the thin portion flat muscle so that what is left of the flat is about uniform in thickness. That thin portion of the flat muscle will just burn up and be no good to eat so trim it away and I repurpose that good flat meat to stews, braised beef, roast, or whatever I need some long cooked beef for. It's also amazing as burger grind!
Because of this I always look for briskets in the 15 pound or higher weight since the flat will get trimmed a bit. Bigger briskets behave better :)
This picture will help explain what I mean about the flat muscle trimming:
View attachment 661775
Finally, know that a brisket is never ton by time or temp. Only when it is tender. You know it is tender when you can stab it ALL OVER with something like a wooden kabob skewer and it goes in with like no resistance.
You can use temperature to tell you WHEN to start checking for tenderness. The thickest yet center most portion of the FLAT muscle is the best place to put your temp probe. At about 198-200F you can start checking for tenderness ALL OVER and then pull when it it is tender all over.
A beginner mistake is to put the temp probe into the POINT muscle or the only check for tenderness in the bigger POINT muscle. The point will always be tender and fool you. The FLAT is the one that is the problem and needs to really be checked for tenderness.
I hope this info helps :D
