I don't find €3000 as a monthly installment all that unrealistic with that salary. We signed a rate of €2000 at the beginning of last year with a net income of €5500. Just like you, there’s still almost €3500 left for living expenses. We're basically trying it out right now since we're already paying over €500 in interest and the €1600 rent still applies, of course. You can definitely live well with that, and a flat-rate holiday for €500 per person is still doable. Then it will just be Crete, Mallorca, Turkey from Check24 or something like that. Especially with the bonus payments you apparently still receive.
However, we don't have children yet, so it will probably be a bit tighter overall than it is for us, but it doesn't sound that far-fetched. Keeping a good household budget and not spending money unnecessarily on "crap" is always wise anyway.
The big problem I see with you is the relatively old house. We are currently building completely new on a 350 sqm plot, and maintenance and additional costs will be as low as they can be with a house for some time. Even if you manage with the €150,000 renovation, which can always increase, with an older existing property, costs can of course always arise. Additional costs will also be considerably higher. Especially installing a gas heating system right now can go completely wrong financially. It is foreseeable that a big CO2 tax will be imposed in the next 10–15 years, and you will want to keep a new heating system for 20–25 years, after all. So if you're building new, do it properly right away.
If you can clear these worries with a good expert and make a good plan, it can certainly work. I don't see the problem with the rate, but the age of the property could become an issue.
Be aware, though, that such a large house with such a huge plot is also a lot of work. Once the kids are old enough, the whole family has to be involved, otherwise you’ll burn out.
It’s also worth mentioning that you’re just as young as we are. The salary will definitely rise over the years, making things easier. For example, mine increases annually as an inflation adjustment. You can work well with that. I have read here that you have a collective agreement. Those also increase steadily if I’m not mistaken.
The OP already has to pay over €2100 in interest alone at the beginning, which is more than €100 higher than your total monthly payment. That’s depressing, at least for me it would be. You actually earn very handsomely and first give a third of your salary to the bank just for lending you money. As your "own investment," you only have the €900 you pay down with. Of course, that ratio shifts more and more in your favor over time.
Then there are probably fairly high additional costs (have you checked how high the property tax would be on the property after the reform?), insurance and cars have to be paid for, and 3 kids cost an average of €365,000 until they turn 18. And suddenly your handsome salary is gone, and you have to figure out how to make ends meet.
When you’re old, you have a house that’s become old again, maybe paid off, maybe not because you had to do another big renovation, and a meager pension, even at maximum pension. When you look back on your life then, you think, for having earned really well, I haven’t really experienced much, where did all my money go!? The answer then is: the bank.
Don’t get me wrong, you can do it, but whether it’s worth it is more than questionable. It wouldn’t be worth it to me.