I found FRA and the lounge rather souless.

TXL is far from perfect, as you pointed out, and the duty free "shop" after security is a joke compared to those in FRA. That said, the extra security and miles to walk are not great at FRA.

TXL does get very crowded, especially from the BA check in through the airport to the lounge.

The FRA lounge does have some windows on one side, but I do not think they let in much light because of the surrounding buildings. Food is woeful at FRA and marginally better in TXL. Yes, the toilets at TXL are on the other side passed the Starbucks, but that is not too far.

If travel was not an issue, I would choose TXL. I have always found the staff pleasant and keen to help.