Not so much through butter, but what you should be checking for is the "resistence" when you put through. If you do it one time and it's TOO tender, next time take it out before the probe resistence is the same. That make sense. There are many degrees of resistence and I can't teach anyone in text how to tell. You use this test as the guide, but you'll have to judge whether it's through butter, or caramel or whatever.
The probe through test is the ONLY method to check for doneness. Temp is not reliable, but it will get you in the ballpark. One might be sliceable at 185 and the next might need 195. Basically no two cows are the same.
I don't know of any Competition cooks who don't probe for doneness as the preferred method and since they're competing for money, must be reliable. And my contest briskets don't come out dry (or I wouldn't make any money at it) but I'm cooking packers with enough fat.
But that's my opinion.
The problem is you're smoking a flat. It has little fat most likely so it will dry out. without adding fat (some use bacon) or adding moisture (the foil) it will likely dry out. Small flats are really hard to do right. If it has varying thickness, you will get some more done than others, so look for more consistent flats when you're buying.
For me, I'll do a select packer with fat before I'll do a flat with no fat.