From a maximum security perspective, you should be checking all the code you install on your computer. No matter if it is foss, audited by some group, or proprietary (if possible). What would stop a bad actor from auditing malicious code and approving it?

As for sandboxing, there's multiple options, not the least of which is containerization.

Again, security is a compromise. More security normally comes at some cost just as less security does.

But back to the topic of the post. You are complaining that SimpleX doesn't work when installed though a flatpak (because one doesn't exist). So perhaps it's not a good software to rely on flatpaks for. Unless you choose to only install software via flatpaks, to which I'd say that's admirable but also perhaps needlessly limiting. Either way it's your choice, but I would suggest some open mindedness of options that may let you use the software you want.