Cost of self-managed homeownership

The prefabricated house builders who work with timber frame construction,
basically don't do anything different, do they?

Hey,
I want to build a house in about a year.
My budget for the house is about €100,000.
I have done some rough calculations and would like
to hear a few opinions on it.

The house should have a floor plan of 7 x 14m and be about 4-5m high.
This gives me an area for the:
Walls: approx. 200m²
(Pitched) Roof: approx. 100m²
Floor: approx. 100m²

For the costs of the walls in timber frame construction, I calculated as follows (price/m²):
KVH (60mm x 230mm): €13 (for 2 running meters/m²)
Klemmfliess Knauff ecose: €7
Vapor barrier Tyvek: €3 (inside and outside)
Wood fiber insulation board: €8 (inside and outside)
Gypsum plasterboard Knauff Diamant: €28 (double on inside and outside)
Putty, primer, paint: €4 (inside only, plasterboard painted directly)
Wood facade larch rhombus: €20 (with battens and additional vapor barrier)
Hardware estimated: €20

Total: €103/m²

The roof will be different but overall similar.
For the roof, I estimate €110/m²

The floor is also meant to be the slab.
Without screed, but insulated. I have already considered
a Swedish slab, but I am not sure yet.
According to the "internet," the m² floor slab with insulation costs about €100/m²
at 25 cm thickness, although I budget €7,000 for the slab with my own labor.

Windows & doors: €4,000
Drywall inside with paint and primer (€15/m²) x 180m²: €2,700
Installation of gas, water, electricity with heating system: €10,000
Machine costs, insurance, etc.: €2,500

So the materials for the house:
€57,100

The grid connection should cost about €7,000.
The structural engineer is in the family.
I would like to avoid an architect if possible. Purchase incidental costs €10,000.

That makes €74,100

I have certainly forgotten quite a few things,
but these should be covered with the remaining €25,000.
I take labor from my family.
The floor plan will be very minimal and open.
The ceiling height results from a mezzanine and gallery.

What do you think? Too naive? Forgot too much?
Why is the house so cheap in my calculation?
Suggestions for improvement?

Thanks for your answer! I am personally still at a very early stage in orientation and idea finding. Unfortunately, my budget does not allow more, and I don’t believe banks finance self-built houses because there is no warranty. The rental apartment I live in does not have larger windows; I feel quite comfortable, maybe I can save energy through size, which is not due to top quality, although nothing is set in stone. I could also spend 25 percent more on all items. Actually, I do not know the exact official process yet, but I hope the structural engineer in the family can help me with that and also create the plans for me. Otherwise, I really appreciate any help and constructive criticism. Personally, I find it a bit unfortunate that one is met with so much "frustration" immediately through off-topic comments like the thing with chips and beer.