LH only releases F within 2 weeks of departure, and they also frequently release more J within 2 weeks of departure as well. Same for LX. If you're flexible, I'd book whatever is available now and then check every day within 2 weeks of departure what you can cancel/change to. If you make a flexible booking, there's no fee to cancel or change. And once your preferred routing is available, either cancel the flexible and book a J lowest reward, or you can change over the phone from Flex to Lowest while you're calling to change flights.

E.g. I had a BOS-MUC-ZRH J flex booking and 3 days before departure BOS-ZRH became available, so I cancelled and booked BOS-ZRH in J lowest. If parts of my itinerary were already set in stone and not available anymore (if, say, I had a ZRH-SIN flight afterwards that wasn't available), I could've called in to change BOS-MUC-ZRH-SIN to BOS-ZRH-SIN and asked them to change the award from J Flex to J lowest at the same time, for a free change and refund of the miles difference between Flex and Lowest.