Its good to have so many wise people to "peer review" my plans. Appreciate all the concerns!
Allow me to summarize all my replies in one post.
Thank you for tell me that singles can only buy BTO one time. I didn't know that.
If so, I agree its better not to rush to buy and sell just to earn a small profit.
Indeed, it is better to keep this chance in reserve. At worst, if I run out of money, I can downgrade to a 30-year 2 room and free up some capital.
Regarding semi employment at my current employer, they do have such a scheme, but its only for unavoidable circumstances, and for limited time only.
I guess if I jump through some hoops, I can get part-time for one year, but after that need to revert back to full time.
Also, I don't enjoy my work at my current employer much. Part-time also impose some limits on my travel plans.
I am open to doing project-based contract work for my company post-retirement, but I will not bank on them giving me business, because they have a tendency of preferring to keep things in house.
I cannot rule out that in future I may change my mind and go back to work, or even get married with children.
My thinking 20 years ago was very different, and perhaps 20 years from now, my future self will have very different ideas.
What I do know is that right now I want to retire, and I will try to plan it so that I don't run out of money even if I stay retired throughout.
If like you say I get bored in future and decide to go back to work, that's fine.
Or I find the perfect partner for whom I am willing to sacrifice my freedom and go back to work, that's OK too.
But I don't plan to hold my breath (stay employed) just on the off-chance that I may find someone in future.
I calculated my expenses before and after retirement, and coincidentally, it is about the same.
After retirement, I save on taxes, some travelling and food expenses, but that is offset by more leisure expenses.
I agree about the need to be careful of one-time expenses.
I already build that into my estimate, but I cannot rule out that I might have miss out some items. Indeed some buffer will be wise.
Regarding why I would be bothered about low pay after retirement, I guess that's just my personal character.
I dislike the thought of being paid less just because I am retired. I feel a person should be paid based on ability.
If I find a worthy cause that I feel strongly about, I can consider low or no pay.
But so far haven't found such a cause.