Multiple simultaneous streams / categories / databases of notebooks

I checked, but as far as I can see, this is not a duplicate.
As others mentioned, there is a need for different notes categories (like work, private, etc.). For confidentiality and legal reasons, it would also be important to separate the related databases. Just like one would have multiple accounts within a single email program.
I understand that profiles are closely related, but different. And switching back and forth basically amounts to closing the program and reopening, which prohibits use cases like copying between different accounts/profiles.

As far as I see, the closest is to have multiple separated profiles and run multiple instances on them (however, this seems tricky to get reliable.)
Are there better ways? Would there be interest in having such a feature on the roadmap (and how realistic would it be).

For me a key aspect would be to have multiple separated databases, so that I can archive different projects separately. (And have this from the beginning, so that these are never stored in the same place.)

I am not sure your demand / need is very clear ... clearly expressed, well defined ...
If confidentiality is a concern, then "easy" copy/pasting from one profile to the other may not be such a good idea. In that case separate profiles (and the restart required) may just be the price you have to pay. If there was an easier way (two instances of Joplin running in parallel, or Joplin handling both databases in parallel at the same time) it's almost certain there are spillovers, caused by the users ... sooner or later ...

There is always a tradeoff between different levels of confidentiality.
If you want to have it perfect, you need to use different computers.
However, every mail reader provides these days the capability of having multiple accounts / profiles simultaneously, even though the different accounts should typically be kept separated, but every now and then, being able to copy messages in between can come in handy and looking into both of them at the same time, can become extremely useful, that's why email readers provide this.
A relaxed version of this is: having different notebooks for different projects. (perhaps with different customers). The need to restart everything just to have different synchronization targets is a limitation. (Imagine, you need to just look up in your general notebooks, when switching to the project notebooks

But I understand that at this point it is not possible. (And I am afraid, it may even be impossible with the current architecture to realize.)
This is why I posted it as a feature proposal.