Hi there and welcome!
I think the guys have you on the right track.
The main thing is that the brisket is done only when it is tender, never by time or temp.
It is tender when you can stab ALL OVER with something like a kabob skewer (I use wooden ones) and it goes in with no resistance ALL OVER.
Next, temp will help tell you when to start checking for tenderness BUT the best place to put the meat probe is in the thickest yet center most portion of the FLAT muscle.
This spot is hard to nail so I always use 3 probes coming from different angles and 1 usually gets it right... the one reading the lowest temp at the end hahaha.
I usually check for brisket tenderness around 200F Internal Temp (IT) of the meat.
When I do a Prime brisket I check at 198F IT because it is fattier all over and gets tender faster than a Choice or Select cut brisket.
If not tender ALL OVER then let IT rise another couple degrees and check again. Repeat until tender.
When it comes to resting I agree that if you are using a cooler to let the temp of the brisket drop some before putting it in the cooler. I just tightly wrap in 2 layers of foil and then wrap in 3 bath towels and set on the counter. Then 4-5 hours later it is piping hot and ready to slice, serve, and eat.
Finally, lets talk brisket timing.
Again a brisket is not done by time or temp BUT you need to know when to start a brisket so you can have it ready by the time you want to eat.
I smoke briskets unwrapped the entire time in my rewired PID controlled MES40. At a steady 275F smoker temp it roughly takes me about 1hr & 5-10min a pound to hit the window where it can become tender. By the way, a brisket does not care what temp you are cooking it at as long as you arent burning it.
So knowing all of that I plan at that hr per pound and then add 4hours to the total time. Why?
Well if you finish 4 hours early then fantastic, you get to rest the brisket and it will be piping hot when ready to eat.
If you don't finish 4 hours early.... you have 4 hours of time to hope it gets done before hungry people attack you with forks and knives hahaha.
So lets sum it all up:
- Brisket is done when it is tender, never by time or temp
- A brisket is tender when you stab ALL OVER with something like a kabob skewer and it goes in without resistance, keep checking until tender all over
- Put the temp probe in a brisket at the thickest yet centermost portion of the FLAT muscle and understand it is a difficult spot to nail so you can easily get high readings that mislead you because you may have missed the sweet spot a little. Hence tenderness checks :)
- Let your brisket cool a little bit before putting in a cooler
- Timing, timing, time! Figure out a reliable smoker temp so you can get your timing and planning down.
- I didn't talk about this but... tweaks and improvements will put your brisket over the top. Once you get these other areas down then we can talk about trimming a brisket, wrap vs no wrap, etc.
For example when it comes to trimming, the thin portion of the FLAT will burn up on you so I always trim it away and repurpose it. You will learn more about these other areas as you cook a few more. Once you nail the points above, improvement tweaks on a brisket will yield a better result in the end and avoid wasting good meat :)
Welcome to the world of brisket! Once you really nail one you will truly understand why it is the king of smoked meats :)