I grew up in a situation like this: my parents built and we had to watch every single mark. My mother was a strict financial manager, with my father sometimes asking at the end of the month if there was still enough left to buy a sack of cement.
So really tight and a lot, almost exclusively own work.
So I know that it works, and my father was also someone who really enjoyed doing such things, was very skilled at handiwork, and beyond that had a few other builders around him in the same situation, and they always helped each other out. Which was very helpful for tasks where more than two hands were needed.
I want to point out a few things:
In my opinion, the ground slab in my parents' house is botched. My father was obviously taken for a ride there. He, of course, searched for the cheapest possible option and my mother had already noticed back then that something was not quite right, judging by the behavior of the person who poured the slab for us. Ultimately: the basement floor is always somewhat damp. Despite my father having read extensively and learned a lot - he did not recognize this and could not make a complaint due to lacking expert knowledge.
Lessons learned: you still need someone to look at it professionally if you do everything yourself.
The house is still standing, no question, and will be for a long time - but eventually we’ll probably have to address that.
My father worked hard and long for years in addition to his normal full-time job. Back then with far fewer technical aids. Much was still dug out by hand (renting an excavator was too expensive - only the building pit was made that way), he built the house himself (thank God Ytong, it was easier than bricks), the concrete did not come with a mixer, but from a concrete mixing machine and by wheelbarrow, etc. Afterwards, he was physically worn out with problems in the discs and his back. Had he not built the house himself, he surely would have had fewer aches and pains.
We moved into a half-finished house; paying rent and already servicing the loan was not possible. As a child it didn’t bother me, it was rather exciting. But when we children were sent to bed, my parents tiled three rows in the dining room every evening. By morning they had set, you could step over three rows, and until we children were back for lunch, it was walkable. The plaster came much later - we lived in darkness for about 2 weeks because the blinds were down. Thankfully, the bathroom was finished, the kitchen not yet, it all came step by step. As I said, as a child I remember this as not bad but rather exciting. How I would have experienced it in my parents’ situation - I think they were not quite so relaxed...
When we were finally really finished, my father said that NOW, with all the knowledge he had gained and without time pressure, he would like to build another house. He would do many things differently, better, because he now has a completely different level of knowledge than before the build. Then I think of the saying that you build the first house for the enemy, the second for a friend, and the third for yourself - there is certainly some truth to that.
Not to forget (I already mentioned it at the beginning): there were always helping hands, even if you quickly needed someone because some materials were delivered and had to be taken off the truck as quickly as possible (no, back then a transporter was not necessarily equipped with a small crane or had a forklift along). So you quickly went to the neighboring construction sites, there were always some and they were also willing in such a situation to drop everything and help quickly. Since that applied to everyone, it was no problem. At least those who calculate with a lot of own work have this. But in my opinion, that is an item not to be underestimated!
I really wish you much success (aside from the weaknesses of the room layout), but I am really skeptical about the budget and the wishes. My little essay here is perhaps meant to help you accept reality a bit more. A father-in-law alone, no matter how great a handyman he may be, simply cannot cover everything.