I have the same concerns about installing flatpak browsers. You apparently lose integrity of tab isolation. At least nobody seems to know if you do precisely, but it’s different and the developers don’t seem to trust it. I’m running Brave Flatpak now and it works great but e.g. the Layer 1 sandbox should say Namespace and be green if it was sandboxed as developer intended:
I think installing it in a distrobox and then exporting it from the container so you can launch it easily without distrobox commands is probably the most optimal (to get intended browser process sandboxing at least). A couple downsides I see is no automatic updates (but just a couple clicks in DistroShelf) and if you were to launch Brave and then close it, the container stays open consuming resources unless you manually close the container. I don’t remember how much it uses but that’s easy enough to test. Maybe some scripting can take care of that for you. Or if you have other packages you need you can install them in the same container and it won’t matter that it is left open. But if you have plenty of RAM (and I probably do as well so I don’t know why I care) then it’s probably a moot point.