Does anyone have advice on the check-in and tax refund office timing at Berlin Tegel? I am trying to think if the below would work:

I see that the refund office is open beginning at 6 am. I have a flight departing TXL-LHR (then LHR-DFW-SFO) at 7 am. I can probably carry on my tax refund items. Might it be feasible to go to the tax refund office at 6 am and still feel comfortable about making the 7 am flight?

Am I mistaken, and actually I will need to get the stamp at Heathrow?

The last time I tried this was at Malpensa (Milan), where the Italian woman manning the tax refund desk appeared, stunningly well-dressed, and espresso in hand, about an hour after the official opening time, giving me just enough time to sprint to my flight. I figure things will be more timely in Berlin, though.