I'be done the loop in FRA in less than 15 minutes, during the morning. Since you get in from IAH that should be similar. The afternoons are busier since many Asians will queue.

What I did is buy case at FGS in baggage hall B, go up to customs (counter 642 I believe in check in hall B) and then back down to get the 19% back. But in your case you may have to do the customs bit at the final EU port, and I don't know how FGS handles refunds remotely.