International UA F bears no relationship to domestic F. They have suites that are long in the tooth but convert to true-flat beds. Food and bev not a huge improvement from international C. (No Dom or Krug.) I think if you can sleep in the UA C cradle seat it's not worth it; if like me you need a flat bed to sleep then it it probably is.

Have you consdidered rebooking your BKK-FRA segment on TG? You can upgrade for cash on the day of departure for about 20,000 baht (about 500 USD). TG is reconfiguring their aircrafts, not sure if BKK-FRA has new cabins but I've flown old style TG first and it's not bad with flat seats and plenty of Dom. (You wouldn't be able to use the LH first lounge I guess but you'd save a bunch of miles.) New style is supposed to be quite nice with excellent IFE.