This all somehow seems very familiar to me.
I am also single, coincidentally a civil servant (but with a uniform), and I was also 31 when I built my house. Now at 33, I can say "I did everything right for myself."
In my case, it’s "only" a semi-detached house on about a 300m² plot, and I am financing "only" 200k€, but it is a similar situation.
By the way, my 11 windows (9 floor-to-ceiling and 2 skylights) are cleaned by a company, with frames inside and outside and the porch roof for 55€. I don’t find that worth mentioning, and you don’t do it every month.
What I absolutely have to admit, though, is that you spend a lot of time in and around the house. Especially in the beginning, even more so when you are landscaping the garden and working on other various projects. After finishing the house, I spent 3 complete vacations and countless weekends in the house and garden. At times I even asked myself if I was still all there mentally. But now that I’m roughly done, I don’t regret anything about the whole thing, except that I wouldn’t do the sanding and filling work myself again.
I have 136m² of living space and about 60m² of usable basement, and I think that’s not too big for me alone. There are also children’s rooms available. Certainly, it’s a bit more work to keep the house clean than a 50m² apartment, but I enjoy doing it. For me, the house and especially my garden have become a real hobby with constantly new projects coming to mind.
I would now set a financial framework for what is possible and what you can afford monthly, and if that fits, I would definitely build.