Hi there and welcome!
Your time estimate is going to be off by a bit if you want to eat by 7pm.
Since you are rushed for time lets jump to what MAY get you to eating at 7pm. Just understand that TIME is your greatest hurdle right now because you don't really have much of it to eat by 7pm so this is going to be very a tight deadline.
- Crank up your smoker to 275F and if it runs a little hotter that is ok because you have way too little time to work with.
Brisket does not care what temp it is cooked at as long as it isn't burning. Turn it fat side towards the heat/flame to help protect the meat if temps spike or you are worried they will get a little hot
- At a steady 275F smoker temp my briskets take around 1hr 5-10min per pound to begin probing tender, though I cook mine naked the whole time. IMPORTANT: Briskets are never done by time or temp, only when tender. Use the Internal Temp (IT) of the meat to tell you when to check for tenderness. You check for tenderness by stabbing ALL OVER with something like a wooden kabob skewer. When it goes in with no resistance it is done.
- Do NOT open the smoker or spritz. You will definitely not hit your 7pm time if you are letting heat out by opening and spritzing. Spritzing will doom you on your timeframe plus it's not needed for a whole packer brisket.
- Your meat probe should be in the thickest yet center-most area of the FLAT muscle. Never the Point muscle. The Point muscle will be ready way before the Flat muscle and if your probe is in the Point it will cause you to think your brisket IT is higher than it really is in the problem spots... the Flat
- If you wrap too early your brisket will taste like roast beef instead of BBQ brisket so wrapping at 165F means you have the probe where I mentioned to put it (thickest yet center most portion of the Flat muscle) and you let it sit long enough to get a good amount of smoke. Like stalling for 2 hours at 165F and then wrapping is WAY better than wrapping the moment it hits 165F. You do not want to spend all that time for it to turn out like roast beef.
- Once you foil wrap it, crank up the heat even more, like 325F+ would be great. This is because you are no longer getting any smoke on it so time to speed it up even more with your tight time frame
- It is done when it is tender, never by time or temp. When the IT of the meat is 200F check for tenderness by stabbing all over and if there is no resistance it is done. If you find resistance (likely in the center of the FLAT muscle) let the IT raise another 2F degrees and then try again. Repeat until no resistance which = tender.
Lots of people check earlier BUT you have like no time to be opening the smoker and letting heat out.
I bet you will have no time to hold this thing but if so, let it just sit foil wrapped and cool off until its about 160F IT and it will be ready to slice and serve. The hotter it is the more it will want to come apart on you as you slice.
This is a no BS set of bullets to help you make this successful because again, you have like less time than what is necessary to pull this off so gotta go hard, work it to the letter and don't flinch.
Ask any questions you have :D