Arch works well for gaming. However, depending on what you're doing, you should keep this in mind:
on any distro, updates may break things or change the behavior of apps. The difference in arch is that youll update no less than weekly on average, maybe biweekly at worst. This would matter more if you have a complex setup. If you're just using steam, I wouldn't worry
arch only uses the latest versions of software. If you ever install something from outside the arch repos, you have to make sure it is compatible with recent versions. Sometimes it may not be.
Why is JSON better than XML? It's more modern, sure, but from technical perspective it is not objectively better right? Not something worth switching protocols for.
You mention XMPP has transports as opposed to Matrix bridges. I thought they give you roughly the same outcome. What's the difference?
Unpopular opinion, but Gentoo is perfect for ARM. Availability of pre built binaries for ARM can sometimes be an issue. Gentoo gives you the option to compile from source, so that if a package is available for x86, it will still most likely work with ARM
Being able to run a distribution on multiple machines does not mean it is free of vulnerabilities. You'd only know if you're checking CVEs for each package you use.
Lots of eyes is not enough. As I mentioned earlier, there are many popular programs found on most machines, and some actually user facing (unlike xz) where vulnerabilities were caught months, years, and sometimes decades later. xz is an exception, not a rule.
There have been many videos posted before that clearly show oil-carrying trucks