I absolutely cannot imagine NOT using aluminum SER for such an application. Copper for that is just plain crazy.
But this guy says he couldn't sleep at night knowing there was aluminum wire
in his walls.
That's because he's uneducated. The aluminum wire horror stories are based upon the aluminum branch circuit wiring done in the 1960's into the 1970's. It wasn't the wire itself that caused the problems; it was the wiring devices that weren't designed for aluminum wire. They hadn't even been invented yet!
I would run aluminum branch circuit wiring today using CO/ALR devices if I could. I know that will never happen since you can't trust that homeowners won't go back and alter the wiring improperly. I certainly have no problem with aluminum being used for ranges, dryers, furnaces or for feeders. That's how my house is wired. Ironically, they used copper for the 300 amp service.