I avoid snaps, don't trust Canonical and they've had malware incidents. There's also a degoogled chromium flatpak if you're worried about Google worming into your system. Idk how on security updates or anything it is though v normal chromium though.

IIRC the flapak packaging system is completely open source while snap does not release the server code.

Also, snaps were forced to the users, had terrible performance at the start, thus, making them unpopular.

Neither isolates everything. Both have some isolation features. The features enabled by default vary from package to package, so you would have to look at the permissions on each package to find out.

For a bit more isolation than a flatpak/snap, I suggest creating a separate user account for running chromium (or any other moderately nosy software). Note that linux lets you log in to two accounts at the same time, each with its own desktop, and switch between them. Check out your desktop environment's "switch user" function.

For even more isolation, you could run chromium in a hypervisor-based virtual machine.

Even better: https://flathub.org/apps/io.github.ungoogled_software.ungoogled_chromium