Paying "rent" to the partner... how?

We do not buy together, that is now settled. It is probably not ideal, that's true, but I accept it as it is. I myself am curious to see how things will continue. At first, I was actually prepared and about to pay the "local customary rent" into the joint fund, that is, what I would have to pay elsewhere anyway, but you have already opened my eyes to that.

Yes, he saves, but he also pays for the maintenance alone. When my current wife moved into my house, she paid me €100/month. Period, no ifs or buts. I do not profit from my partner. There has to be something fair for both, with which both feel comfortable. Different opinions often help with that. I am also paying off the current house. But both of us are involved in the financing, both are on the land register. We have kids, and I was able to study on the side, build a "career," etc., while she did everything at home. Maybe that will change this year. Then I will go part-time and she will advance professionally. Who cares? But WITH kids. They are not considered in the original post. I also wouldn’t want to pay too much. He saves, you pay. That’s unfair, in my opinion.

I already wrote above that if both parties are on the same level regarding income, the whole thing actually works quite well. One has the equity for the property, the other "saves" rent costs through the equity but participates in the actual costs, and for a house these include capital costs (or conversely capital income).
If the saver then spends more on "everyday things," this can be prevented by a joint account with a 50:50 share (or adjusted to income).
The saver can also invest his money in real estate to avoid the risk of frittering it away and benefit from real estate prices just like the homeowner.

As an example calculation assuming only equity is used:
House value: €600,000, interest as 10-year fixed deposit 1% (for easier calculation, realistically also 1.3% possible) -> €6,000 -> €500 per month.
Additional costs: €300/month
--> Monthly costs €800 --> 50% = €400 for each partner
A comparable rental house would cost about €1,400 rent here, + additional costs -> €1,700 --> 50% = €850 for each partner

Seriously? No joint account anymore? 2020?

Why? There is no mine or yours but an ours.
We are not building a house together for 700k plus and then start playing games about who pays what and what belongs to me and what to you. Sorry, am I with the woman to improve myself with money or because of happiness? For me, it is definitely happiness. I have enough money, equity, and real estate myself, and one thing I can tell everyone: some things make me happier than money. And you cannot buy these things with money. Money only makes life easier....

You are right, the "we" and the associated quality of life matter. For us, this also includes allowing each other the freedom to do whatever we want with our money without having to explain ourselves. That is why there is a joint account and everyone still has their own accounts. If my wife spontaneously wants to buy a nice jacket for one of our sons, I don’t have to know what it costs—especially not if I am not in the best mood with him at the moment and certainly wouldn’t want to contribute. Her account, her money, no discussion. I recently bought a new watch; my eyesight is getting worse, and I could no longer read the delicate 24-hour single-hand model that I had enjoyed wearing for years. The price I paid is not something to justify. I don’t have to. I’m happy with my watch, and my wife is happy with me. No discussion. Despite all the "we," there are still two "I"s. We have honored that for over 30 years.

Off topic. What kind of watch?