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Agree with @Rethanis ; however I have some additional thoughts on that gap here: Equivalent of Multi-Account Containers or Temporary Containers Extension (FF) - #13 by JimB1
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I wonder if you could use Brave Sync for this; create a new profile and put it onto the same Sync Chain that your other profile is on, and then detach it. Note that this is similar to what I discuss in #1 if you follow the link.
You’d have to be really careful to know what the ‘merge’ behavior is between disparate chain members though; I think likely Bookmarks and so on would be fine, but I have no idea what would happen where things like Settings and Apps are different. Maybe you could test by creating two dummy profiles, set one of them up the way you want, and then add the other dummy profile and see how it goes. And if you go through with it, please report back. 
- I could be wrong, but I’m pretty sure PWAs stay with the profile they were created in. For example, in my case I have multiple Brave profiles, with different Google accounts logged into them. One of them has a Google Chat ‘app’ installed. Even if I don’t have Brave running at all, if I open the Chat app (PWA), it opens and is logged into the correct account, which would be impossible if it had loaded in the wrong Brave profile.