If you feel underutilized, why not pick up a gig or two? A friend of mine who is a government lawyer teaches a class at Strayer U. There's all kinds of legal gigs you can explore. Write articles. Make presentations. ENJOY what you have. I am a comms officer at the WB with a PR agency background and NO WAY will I ever go back to the world of billables. Never.
OP---as someone who has done big law and in-house (with a life/balance similar to yours in a "unicorn" job I really enjoy)---my suggestion is to take the advice of the PP above. But instead of teaching---go for writing articles and making presentations---if you make enough of a name, then if you decide you really want to go for the money back in BigLaw in a few years, you'll have laid the foundation for building a book of business. Is your UN job in a "niche" type area? I think the big law partners in DC who can command the highest $$ and still have a reasonable life are the ones who are the absolute expert in some arcane area of international or federal regulation which is of critical importance to high-value industries.
And in terms of career satisfaction---there are other barometers of success---having a job that is unique, highly competitive to get, more interesting---all of that can be as satisfying as $$$$. Yes, sometimes it is tough to watch the colleagues who stayed in BigLaw trading up their houses, easily footing the bill for multiple kids in private school, but just remember that all of that comes at a huge cost to having any type of outside personal life.