I think something that's misleading is that you can pretty much pick any stage you want from the start. The problem is you can only beat later stages with resources you earned from earlier stages, so it seems like they should be played in a linear order anyway. You could either have them unlock in a linear order or just cut out the map screen altogether.
Or maybe it would also work to just give players a set amount of resources at the start of each stage. On my last attempt I lost on the final stage, and I just don't feel interested in replaying all the previous stages again for another try. If I didn't need to start from the beginning, or if there was at least more leniency, it would be less frustrating.
Everything else is a matter of rebalancing and polishing what's there, so I'd say expand on this further.