Thank you all for your responses.
SeattleCPA: very useful articles
Radagast: I will also check on Bogleheads. Very useful site. I'm also realizing that maybe my issue might be irrelevant to many here. I thought I'd give it shot.
waltworks: I hear you. I was expecting a face punch or two. We earn and spend a lot. We live in a HCOL area (NYC environs). In our defense, we do save about 50% of our aftertax income but certainly we could do better in containing costs. Regarding our careers, I've struggled with whether I'm in denial or whether I really like my work. I don't think I'm in denial. I have a great, meaningful gig. It may not seem apparent, but I think we are both trying to make the world a better place using our training and expertise. FWIW, I do medical research on brain development and how exposures during pregnancy can derail optimal maturation of the brain. As I said, I've had one project in particular that I've spent 15 years on and I think that in 5 or so years, the results may change medical practice and the future health of a bunch of kids. I also do a lot of mentoring, teaching, and am in a position to help other scientists grow their careers. My wife is an idealist and what is driving her to stay in her work is the possibility to develop a medication that treat a condition without any current treatments. If she succeeds it would help relieve a lot of suffering. Her drug is in Phase III and like me she can see the finish line. Unlike me, her work environment is more toxic, but she doesn't want to quit (not yet anyway). So yes, we are unmustachian in not racing towards early retirement but trying to achieve/maintain FI. Despite our work, we have quite a bit of freedom to work from home and have flexible hours. Our son gets a lot of time with us. We eat all of our meals together as a family. Don't have a nanny and rarely spend time away from our boy. He seems to be doing great. Maybe this all sounds defensive, but we are amazingly lucky.
In any case, I was hoping to get some advice on how to think about bonds in our overall investment strategy. I got some good pointers. FWIW, the mustachian approach to life resonates with me: efficiency, simplicity, mindfulness, vigilance about luxury and hedonic adaptation, etc. I'll keep reading and learning.