I know the feeling. I'm in the military and so my job description changes often. I love my core job when I get to do it (which is more and more rare as I move up in rank), but frankly I hate most of the other work I do. The problem for me is, we've reached and well surpassed our FIRE number. At this point, we just keep adding more and more to our stash almost pointlessly. Problem is, because of our commitment we can't quit for another 4 years.
I understand the four-year commitment and the assignment officer's expectations. But since you're already FI, let me ask a question that I wish I'd asked myself 20 years ago.
"What would happen with your commitment if you resigned from active duty (instead of sticking around for the active-duty retirement) and requested to affiliate with the Reserves or National Guard?"
I doubt that you'll find any guidance on this in your service's personnel or assignment manuals, because nobody ever asks the question. (If it was an enlistment contract then I wouldn't even ask it either. Officer personnel planning is a tiny bit more flexible.) The reflex response for asking the question about a bonus contract is a spluttering noise followed by "...but you'll have to pay back all of that money!"
Since you're already FI, you'd only have to make your FI stash last until a Reserve/Guard pension kicks in at age 60. (At the risk of belaboring the math, the pension is based on the pay tables in effect when you're age 60. You could calculate a High Three pension in today's dollars, using 2.5%x<your current years of service>, and hope that military pay more or less keeps up with inflation until you're 60.) You'd also throw in a small amount of income for "one weekend a month, two weeks a year". Or you could even surprise yourself by discovering a billet that's worth a little more AT or ADSW.
I don't think you could simply submit a resignation letter up the chain of command tomorrow (although that would generate some interesting discussions). If you have a polite relationship with your assignment officer (or any assignment officer, or anyone who's ever done the billet) you could ask them the question. You'd want to know how they'd make it work if your specialty was overmanned or if you were an unplanned loss. (They have to do this all the time for medical or physical disability retirements.) Usually the answer is something like "We'd hate to let you go, but we could if we had to, and the big discussion is recouping the bonus." The fact that you're committing to the Reserve/Guard (and not just going to work at General Dynamics or cruising around the world) would help the personnel staff feel that they'd saved face and honor had been satisfied. They got their money back and the military still owned you until you reached 20 good years of Reserve/Guard service.
Another research project would be finding a Reserve/Guard recruiter who could figure out where you could get a billet with a local unit. There might not be one in your specialty but there might be a number of general-purpose "any ol' field-grade officer" billets.
If you get any encouraging signals-- like the active-duty community managers would appreciate your volunteering to open up more promotion opportunities for others in your specialty-- then it's definitely worth talking with your boss about going to the Reserve/Guard. The command's problem would be how long you'd been in the billet and whether they'd get a relief (doubtful) before your original end of that tour.
The chances of this all coming together are probably less than 30%. The difficulty in assessing the probabilities is that you don't know what personnel struggles the assignment officer is dealing with (because they don't want the public discussion) and because you might not be able to find a suitable Reserve/Guard billet. If you can answer those questions, though, and present a pre-packaged solution to the chain of command, they might actually support your decision.
I'm only bringing this up because I keenly understand the morale erosion of a four-year commitment and the corrosive effect it has on your soul. In retrospect, my frequent bicycle commuting on a 24-mile round trip probably saved my blood pressure.
Of course your XO would immediately give you the crappy jobs for the rest of your time on active duty, and your performance ranking would plummet to the bottom of your ranking group. But the Reserve/Guard forces are used to seeing that from someone who comes to their unit from active duty, especially beyond 12 years of service.
Personally, I applied for the 1990s drawdown TERA program three times before my assignment officer admitted that they'd cut too deep and had to keep everyone, bonus contract or no bonus contract. In exchange, they agreed to continue me on active duty until 20 and to extend me in my billet at a training command for nearly a five-year tour. I still should've resigned for the Reserves, but I was ignorant & scared and the compromise was "good enough". Yet the interesting point was that I actually had a discussion with my CO and BUPERS based on the understanding that I already had enough money and was seeking a better quality of life.
A few years later when my active-duty spouse got into the same issues with her assignment officer, we already knew enough about the Reserves for her to make the choice to resign. Nobody in her chain of command could understand the concept of "we have enough money", and she tremendously enjoyed her time in the Reserves. She even kept drilling because she got a great independent-duty billet.
And of course our whole family reaped the benefits of her quality of life and her happiness.
