If you need more than 18,000 you have almost 12,000 so far. I would ask what FTL is worth to you and whether spending a significant amount to get FTL is worth it.

If you have earned your 12,000 miles by flying lots and lots of cheap 25% earning Y tickets between LH hubs, then the lounge access might be worth it. In that case two return trips in C with a connection at10,000 each will be the best deal.

Otherwise I would spend the money instead on priority pass and a couple of very good meals!