Which task is worthwhile to do yourself?

You will no longer get multiple offers from us.
If only through a relationship
When the work will be carried out is the next question.

You need a lot of time and patience and will become a stalker

My words ...

Otherwise, I would write it out as follows:

1 front door + 10 windows ???

Here we are again with effort + work. Now it's about reading, reading, reading and comparing, comparing, comparing.
Example windows: What interests me as a building owner?
- Compare thermal insulation values (but there are also great explanations in literature and on the internet about what Uw and Ug are, for example)
- Double or triple glazing (well, that's actually clear but possibly for cost reasons for the garage, also depending on the required thermal insulation values)
- How and where are the seals installed? (In the frame or in the window? Which I find also has optical reasons when the door/window is open / 2 or 3 seals)
- Concealed fitting or applied fitting?
- What security level? (e.g. RC1N for us, but corresponds in equipment to RC2, just not certified - accordingly many mushroom cams and security strike plates, ...)
- Which handle (with lock, push button, ...)
- Aluminum, wood, plastic?
- Installation depth
- Manufacturer's reputation
- and last but not least the appearance

Don’t nail me to completeness now, that’s all that spontaneously came to my mind as our decision criteria.
There’s nothing here that absolutely requires being an engineer.

No, it’s not, sorry, with all due respect, that’s nonsense. It takes effort and work, also a bit more lead time. The construction period will also increase by a few months. But you definitely save a lot of money compared to a general contractor. If you get several offers, at least 10% of the construction sum.

I wouldn’t say that so generally. Here, general contractors are apparently not as expensive as in the South. I absolutely don’t see them being underbid significantly here, at least not with a few comparative offers per trade and also not with reasonable effort. And to get meaningful offers or access to certain trades at all, often only contacts help, which one often doesn’t have oneself, but rather the general contractor or the architect.

We also awarded contracts individually.

With individual trade contracts, you can schedule the craftsmen so that, in this case, for example, the screed layer only shows up on site after a buffer of 6 weeks for the plastering work done as own contribution.

Yeah, you can. But the screed layer will probably schedule himself to only have time in 8 weeks. And thus, the house construction will be extended considerably, as it doesn’t only affect the screed layer, but also others.

I can’t live off just talking about offers.

That’s right.

If you get multiple offers, a minimum of 10% of the construction sum.

Unfortunately, that will be eaten up by the extension of the house construction. Interest, rent…

You can’t bring that much own contribution to save that.

That’s why you can’t say anything definitive about it.
With a distance of 100 km, it already makes sense not to have to deal with construction screw-ups. That’s what the general contractor is for.

Here we are again with effort + work. Here it’s all about reading, reading, reading and comparing, comparing, comparing.
Example windows: What interests me as a builder there?
- Compare thermal insulation values (but there are great explanations in literature and on the internet about what Uw and Ug are, for example)
- Double or triple glazing (well, that’s basically clear but maybe for cost reasons for the garage, also depending on the required thermal insulation values)
- How and where are the seals installed? (In the frame or in the window? Which I think also has optical reasons when door/window is open / 2 or 3 seals)
- Concealed fittings or surface-mounted fittings?
- What security level? (e.g. RC1N for us, but corresponds in equipment to RC2, it’s just not certified – accordingly many mushroom cams and security locking pieces, ...)
- What kind of handle (with lock, push button, ...)
- Aluminum, wood, plastic?
- Construction depth
- Reputation of the manufacturer
- and last but not least the appearance

Don’t nail me down on completeness now, that’s all that spontaneously came to me as our decision criteria.
There’s nothing there that you absolutely need to be an engineer for.

But not everyone wants that. Often, there’s no time to acquire half-knowledge.
Yes, there are people who want to support the house project as much as possible with their own knowledge, but you don’t have to.

I also don’t consider this discussion helpful for the OP – it’s about own contribution, to reduce the construction sum a bit. And that just doesn’t work with small things like chasing or the time-consuming task of plastering and painting.
It should also be mentioned that painting and flooring have been done by most people as own contribution for years since these services are not included with the general contractor.

That is of course a basic prerequisite, but you can achieve quite a lot in one day on the weekend.

EL works very well if you really have a vacation or other block time. Doing it just on weekends gets difficult again, because you can't forget the "setup times."

I took 4 weeks off (my father-in-law was there the whole time, my father took 2 weeks off, and my wife 3 weeks; she had to take care of the little one after daycare at 3:00 pm; mother and mother-in-law were responsible for food and similar things)

During this time we

[*]removed old wallpaper, filled in defects
[*]mostly re-wallpapered with renovation fleece (but also accent walls with patterns)
[*]expanded the EL (additional sockets, fitted 10 blinds with electric motors and power supply), and filled the plaster
[*]and then painted (almost always 2-3 different colors per room, and with borders etc.)
[*]sanded and oiled, polished 100 sqm of parquet
[*]new baseboards
[*]installed 2 new glass shower enclosures
[*]replaced sanitary fixtures like sinks and toilets
[*]but also packed everything from the old apartment into boxes

[*]and many other small and big things around it
[*]and before moving in, a thorough cleaning (3 aunts were there who cleaned all day)

From week 3 my father-in-law was running on empty

The actual furniture move was done by a moving company.

We had a project plan, but the times were mostly planned for weekdays (Sat+Sun were buffers). That was partially needed from the second week.
You must also not forget material procurement/selection, as well as rental times for heavy equipment and tools.

But progress was great. There was always something to do.

After we moved in, there were still further tasks; e.g., re-tiling 2 balconies, or similar things.
If you only work on it after work, but on weekends, something like this drags on incredibly long.

Here we are again with effort + work. Here it means reading, reading, reading and comparing, comparing, comparing.
Example windows: What interests me as a builder?
- Compare thermal insulation values (but there are also great explanations in literature and on the internet about what, for example, Uw and Ug are)
- Double or triple glazing (well, that’s actually clear but possibly for cost reasons for the garage, also depending on the required thermal insulation values)
- How and where are the seals installed? (In the frame or in the window? Which, as I find, also has optical reasons when the door/window is open / 2 or 3 seals)
- Concealed fittings or surface-mounted fittings?
- What security level? (e.g., RC1N for us, but corresponds to RC2 in terms of equipment, just not certified - accordingly many mushroom cams and security locking pieces, ...)
- Which handle (with lock, push button, ...)
- Aluminum, wood, plastic?
- Installation depth
- Manufacturer’s reputation
- and finally of course the appearance

Don’t pin me down now on completeness, that’s all that spontaneously came to my mind as our decision criteria.
There’s nothing here that requires you to be an engineer.

Read, read, read

Well, if I were to become an architect, yes! But I don’t want to.

So that means every builder, no matter what education, is mighty enough to acquire all this building stuff in order to communicate on the same level with the craftsman?

No way in life