My theory is that the air leaks along the strip, anywhere, then tracks to the valve. So even a perfect seal may cause a air to come out at the valve. I also think this is why strippers work so well.
I've had luck seating the tape by inflating a tube and tire on it for a day or two before mounting sans tube.
He-man force to screw in the valve stem is assumed.
I stopped using road tubeless. I don't get many flats and tubes and tires are lighter than their tubeless+sealant counterparts. Especially so if you get those crazy expensive orange tubes ( I don't go that far). Gravel and mountain bike though, I'd rather fight at home and have a reliable ride.