I agree with your solutions - it's a balancing act.
I don't see a need for major changes.
Having some more 'forest' backdrops, maybe some 'icebergs,' 'stars' as well, with wild, random, organic elements [even if they repeat] in other levels would probably help with the issue of the 'two-tone' looking backgrounds. I'm sure it will all fall together nicely when more levels are complete.