What budget / financing is possible?

Hello everyone,

I have been lucky to have a well-paid job for 18 months, even though I work in an industry where people usually live hand to mouth.
Now that all my friends and acquaintances are settling down, buying property, starting families, etc., I am also starting to ponder. I never thought I would earn enough money to even own a small condominium, but things turned out differently. Slowly the idea of building a house is growing. I’m sure you feel the same way: the longer you deal with it, the more expensive it becomes... You start getting crazier ideas (like how about a sauna in the basement? etc...)
But I want to get back down to earth and would appreciate your advice.
My specific question is this: I want to find the necessary balance between house financing and financial flexibility. I don’t want to feel burdened by the house. Given my possibilities, I lack the imagination. How big should the house be? At what point would you consider the risk too big that financing the construction project could become a burden?
However, I’m not used to juggling such amounts, so I look forward to your experiences.

Hard facts:
Age: Early 30s
Contract: Permanent
Net income: 4200 €, but tending to rise, as I have been in a so-called provisional service. With a permanent contract in 2021, the salary will rise to about 5200 € net and up to 8000 € net by the end of the career through experience levels, provided all requirements are met (further training, health checks, etc.).
Expenses: All in all 1600 €
Savings rate: 2700 €
Partner’s savings rate: 1000 €
My partner’s income is also expected to rise as she currently only has a doctoral contract and will apply for a “real” position after defending her thesis.
We would finance the construction project together.
Estimated equity until the start of the planned construction project: 125,000 € (building savings contract, cash assets, gift from mother-in-law ...)
No children planned.

I had a casual talk with the bank advisor, and she got excited when she checked my data. But of course, they want to “sell” me money and would finance up to 900,000 euros. That sounds like absolute madness to me!

Our friends have financed construction projects between 300k and 550k. That seems more reasonable to me.
However, I keep seeing typical houses in building blogs and YouTube videos for which I would have to pay the prefabricated house company alone 450k, without additional construction costs, land, etc...

What budget would you set in my case?
Am I thinking too conservatively?

Totally lost
Best regards
Stephan