

secureblue includes modified images of CoreOS called securecore. While this doesn’t fix the issue you described, it is worth mentioning as a (technically) more secure option than both Debian and CoreOS.
Honestly I would not recommend securecore or secureblue for security. Small team, no track record, very little funding. I doubt their patches are audited by third-parties, and their userbase is probably so small that bugs are not found quickly. I’m sure you’ve already seen this PrivacyGuides thread on secureblue but the project is still very unstable. Their ideas may sound nice in theory, but patches can end up introduces more vulnerabilities than they fix. There are going to be breakages, changes in recommendations, bugs, regressions, and all of these impact security. I would not recommend it until their userbase is larger. You might ask how their userbase could ever get larger by my logic, which is why I’ll say that I’ll only recommend it for users who care about contributing and supporting the project, and improving the security of the future, even if it means sacrificing a bit of their own security at the present.
From my experience, having a large userbase and strong track record are the most reliable indicators for good security. You can always find articles criticizing old projects for security issues, but that’s simply because new projects aren’t under the same scrutiny (GrapheneOS is a rare exception). This is why I recommend Fedora Workstation/Silverblue over secureblue, Debian over CoreOS and securecore, and F-Droid over Accrescent. Though if you want to fight for a better future and test drive the hot new stuff, all the power to you.
It doesn’t have to be, but if all Linux apps were standalone binaries, installing apps would be a PITA. Flatpaks have better integration with the desktop environment (like automatically handling desktop shortcuts), can share runtimes to save space, have a standardized way of handling permissions and launch options, etc. The Linux world is moving towards flatpaks for many reasons, and the Tor Browser flatpak is marked as official from the Tor Project. Wouldn’t it be reasonable to expect it to work, and to get some sort of notification if it breaks?