You haven't mentioned your age - that's an important factor in striking the balance. Yes, you can keel over at your desk at any age, but it's a lot more likely if you're 60 than if you're 35. The older you are, the stronger your FIRE strategy is, because your government retirement benefits will be larger and you are closer to receiving them, thus you can feel more confident including those in your strategy. Also, if you're older, your FIRE plan simply doesn't have to last as long as it would if you were younger. Higher likelihood of dying or becoming infirm + higher confidence in your FIRE plan = pull the plug sooner rather than later.
Regarding going part-time: that can seem like heaven on earth to someone who is putting in 40, 50, 60 hours or more per week in a soul-sucking desk job. If I had had that chance four or five years ago, I probably would have leapt at it. But viewed from the perspective of someone who is already FIREd, it doesn't look so hot. After being FIREd for a little more than a year, I can't imagine going back to work for even 20 hours a week. I have zero interest in being somewhere and doing compulsory tasks on someone else's schedule. I absolutely love the freedom to do whatever I want whenever I want. Even after reclaiming 50+ hours of my time per week, I find it difficult to do everything I want and need to do. Part of that is because I refuse to go back to hectic "busy, busy, busy" mode. Just slowing down to a more leisurely pace allowed everyday activities to suck up much of my reclaimed time. So with the benefit of hindsight, if I were given a choice between doing OMY to achieve a solid FIRE vs. doing five more years at part time, I'd probably suck it up and do the OMY.
