Bathroom floor joint waterproofing protection

It is said about flat roofs: they last 15 years, then they have to be redone.

Who says that? Because that statement is simply nonsense.

Our bridges in Germany are mostly made of concrete (like the dome in the Pantheon), need to be renewed and aren’t even 100 years old.

and I like the idea of building for eternity.

It’s all a question of use and maintenance. If I take a bridge from the 60s, then it was built for the loads of the 60s. If you buy a pair of shoes and only want to wear them on weekends, then you shouldn’t be surprised if they are worn out after the second marathon and three months later.
Houses – no matter how they are built – last forever. Just walk through “old” towns. These houses have nothing to do with modern things and still don’t get “old” because they are maintained.

My question aims at whether the paste would technically work.
Probably only someone who has already worked with it can answer that for me.

You won’t find anyone here, because no one uses this paste for private use. And why should they, it’s simply not needed.

I tried to find a technical data sheet. Unfortunately, there are only certificates in Czech on the homepage. So at least I would want to check whether it really has all the properties you expect from it and not maybe some additional properties that would even be harmful for your intended application. Based on the application examples and photos on the homepage, I would say, better not. It all looks like significantly rougher applications (shell construction). Possibly the swelling effect is even too strong and you might burst the joint and the tiles off.

Addendum: So I believe what you want: durable low-maintenance sealing is fully achieved by a liquid plastic sealing. The only thing that can compromise it is poor workmanship and UV radiation. The first you have at least partially under control. The second does not exist in your application case, as there is another layer above it and hopefully the sun will never shine underneath there.

whether it really has all the properties you expect from it and not perhaps additional properties that could even be harmful for your planned application.

yes, that's exactly what I'm trying to find out. As it currently looks, the "Quellpastentest" will not take place because I have too little information about it so far (I am still asking around with "other people" - so far without results ... )