My latest thought is starting with all premium seating and pricing about 10% higher than defaults.
This will be a quick path to BK. What have you tried so far, and why hasn't it worked?
As long as you find an airport that has a reasonable number of slots to use, it shouldn't be that much tougher than starting on day 1. It's 5 years in, so the used market should be starting to be a bit better. The key is to be efficient, to make good decisions, and to put in the effort to plan ahead. Start on the really big routes, only a couple of times/day, that's where you'll get the fullest planes to start with. Look at the used market, decide on the 1 or 2 fleets you will use to get up & running (so at least first 20-30 planes). You want them to be an appropriate size, and both available & affordable. That may be older 737s, or widely available turboprops, or medium jets. It's going to be much tougher to start with LH, because the planes are expensive, the routes support less planes/day, the operating costs are higher. Basically, research which fleets you can start with. Research which routes you have available. Make good choices with both of those, run an efficient schedule that gets the most revenue out of your planes, and you should succeed. Do not use premium seats, do not increase prices at the start. You can't reconfigure your first two planes until after they arrive, but you can order 3 more planes in the first week (if you start with enough cash), and you'll probably want to reconfigure those to add some business seats.