Construction costs are currently skyrocketing

Of course, in practice most homeowners do not have any _liquid_ assets in retirement that they can live off, because selling the house is out of the question

A stupid vicious circle, because a single-family home for two people, which for example was once built for four people, naturally ties up any available liquidity. Maintenance, renovation, refurbishment, and eating, traveling, driving, etc. are things you might still want in old age.
Thus, your own property quickly becomes a burden. And it also shows that, of course, in addition to your own property, you need at least one building block that secures liquidity.

as a tenant, you are not responsible for the maintenance of the apartment.

You are so cute. Do you really believe the landlord pays that out of his own pocket? You pay for it with your rent. Because he also builds up reserves. You pay off the place and for the maintenance. If he had to pay extra, he wouldn't rent it out.

because you want to "improve or beautify" your place.

The tenant pays for that too. Through allocated modernization and own contributions like painting, for example.

The garden is an example where you can financially stretch yourself every year again.

Gardening is a hobby and a way of life. Why else are there so many allotment gardens?

the kitchen is replaced because at the beginning one could or wanted to afford only a cheap one.

So does the landlord regularly pay for a new kitchen? I don't know that from my time as a tenant.

After 15 years, the rectifier of the photovoltaic system is at the end, a new one is needed,

Shouldn't that be covered by the profit from the photovoltaic system? Nice how some think you get something for free as a tenant. Only if you receive citizen’s income.

It remains questionable to me. I don’t know how old you are now. But you can relatively easily enter the invested equity plus maintenance costs into a compound interest calculator, so you get a rough overview of the capital then available.
I did this myself recently in another thread on the side and ended up with over 400k in assets. That is also correspondingly liquid. Of course, I left out many factors like inflation, possible adjustments, etc.
Therefore, I do NOT believe that you come out much better as a homeowner IF saving and investing are done consistently in comparison.
YES, an investment in well-being. But often nicely calculated in financial terms.

We did not contribute any equity at all

I therefore do NOT believe that you come off much better as a homeowner IF compared on the basis of consistent saving and investing. YES an investment in well-being. But often financially calculated optimistically.

What exactly is the goal of this thought - in my case, I am Swabian: "save save build a little house." And what is the plan as a renter? "save save" - but for what? To be a rich retiree? Does this rich retiree ever say to me "look here I have 400k€ and you don't. I can afford the nursing home!"? During the time of "save save" I couldn’t even invite a sibling + family = 4 people because there was no space with us (3 people). The thought "oh I save so much compared to a homeowner" did not comfort me! Again the final question, what exactly does a renter save for? (He is not allowed to buy a Porsche either, since he is supposed to save consistently).

best regards

One thing is saving, the other is getting the maximum out of your money.
In the end, it all gets spent anyway.
Whether now or in old age

One more time the final question, what exactly is a tenant saving for?

You don't take anything with you on the last journey.
I also prefer to enjoy our little house for as many years as possible. True happiness exists only in the here and now.
I always have to think of my poor uncle, who died alone with all his saved money, and in the end left an inheritance to a nephew he might have seen once every five years. :/