Tips for a single-family house built in 1976

Ten years of peace? Oh, with 83€/m and the tight budget that will be tight. Personally, I would rather pinch a bit more in the kitchen. That is cheaper than 10,000€.

Then:
Gut bathrooms yourself and straighten with plasterboard. Tiles only in really wet areas, otherwise wallpaper. Have the water pipes checked. And if "so-so," have them redone. Same for wastewater, although that is probably still OK.
Then after consultation, only have the rough installation done. Buy items at the hardware store and install yourself. An exception might be tub/shower.
Obtain tiles yourself after consultation in outlet stores or similar.

Have the electrical system checked. Possibly (if 3-core) you can separate the rooms by circuit breakers with little effort. FI switch indicates partial renovation.
Then expand after consultation. More sockets, etc. You can make grooves and set boxes yourself. The rest by a professional! Electricians allow do-it-yourself work.

Have the heating maintained and have it checked whether the radiators are intact. The pipes are often extremely unproblematic.

Windows: Get a carpenter. If the frames are OK, then new thermal insulation glass can be installed. Significantly cheaper than completely new windows and almost as functional.

Do the sloping ceilings yourself. Max. 2000€ and the matter is done.

Blow-in insulation if there is still money left.

I come to my answer already from the question. The OP did not ask a professional, says in the opening that the budget is limited (admittedly like most) and only wants to have what can be seen done. And since a lot still comes from the year of construction, some things have to/should be done at the same time (have). It was a guess from my experience, which was confirmed. It is still possible then. But not comprehensively, rather limited. Let's make the best of it

Sounds like a plan.

The professional should take over. Can also help with price determination. I would also find out whether the ownership transfer possibly triggers renovation obligations. You wouldn't get around that.

And get used to the idea of running around more often in the future in overalls and getting dirty, possibly throughout entire vacations. That saves the most.

... in connection with the craftsmanship, however, and in this regard I share your suspicion that the OP might not be a Goldcard Hornbach customer ;-)

Generally speaking, what you are "short" of in money, you have to make up for "too much" in manual skill. Not all marriages can withstand permanent construction sites.

Windows: get a carpenter. If the frames are okay, new thermal insulation glass can be installed. Significantly cheaper than completely new windows and almost as functional.

Frames and fittings are already a considerable part structurally, and with the seals (with warped frames non-porous seals are correspondingly less effective) are also definitely involved in thermal insulation.

In addition, triple glazing is not only thicker but also heavier – the frame and its fittings must be able to hold that. The small strip for the extra thickness alone does not do it. Nevertheless, I basically agree with you, since the glass surface is more relevant for heat transfer than the frame profile (and wood does not have bad values there either).

whether a change of ownership possibly triggers renovation obligations.

... unfortunately can be the master question whether the budget is sufficient.

We once carried out the beading and reglazing in an office building with mahogany windows from 1979 and Roto fittings from the same year. Almost 16 for a really large object with more than twenty windows, plus new Teroson sealing gaskets, that brought a lot. But the windows are really okay there too, treated with Owatrol every three years, not a single black spot. Good, dense, slow-grown rainforest mahogany. Karsten