Sorry, No First Class lounge benefits.

The Lounges are departure lounges, and your next flight is in Biz class. Also, you won't gain access to the F lounge, because for domestic flights there is no F lounge, just the domestic Business Lounge. these areas are separated in Bangkok.

The only way to gain access to the F lounge would be if you were connecting onto a THAI flight in which F wasn't offered (say SIN, DPS, AKL etc).

Enjoy the F lounge for your return flight BKK-FRA

Cheers, TK