Go to a good notary - even though a top exam is required for this profession, there are also quite a few idiots there. So check if the notary you choose doesn't have bad reviews (we could have saved ourselves a lot of trouble, money, and time).
Otherwise: you can complain all you want, the house currently belongs to your father. Period. Stop discussing whether you could have convinced someone or if you were advised incorrectly back then, etc. It belongs to your father. If I read between the lines correctly, your father is definitely not willing to throw money to the "cutthroat" notary and/or tax advisor – instead, he asks (free of charge!) at the tax office. But that won't help you today, and back then the austerity apparently didn't help in the long run either. I am convinced that the tax advisor told you at the time that you would save money CURRENTLY and from then on everything seemed clear. I can well imagine that the addendum "but please note that..." was not paid attention to back then.
So: put your money where it belongs, get good advice (why do you want it for free?), find a solution and please definitely keep in mind that the problem won't be the inheritance (inheritance tax aside), but really the case that the father becomes in need of care and then the social welfare office reaches out.
I would divide the property appropriately, also clarify the preemptive right. Then a gift of the part on which your house stands. Unfortunately, the house you built will then be included in the exemption, but with a clever division of the property it should be possible for you to stay below the exemption limit. If another division is also under discussion – what was it with the rear part of the property? – then do it all at once; in the long run, it is cheaper than starting over in five years. Include everything at once: right of way, sewer, how long it should last (for example, this can be tied to the father's lifetime).
But you will not get around having to put money into it. Otherwise, someone else will take the money in a few years – and personally, I would like that the least.