Arch is an OS that’s quite fully featured

Not entirely sure what you mean with that.

suited to veteran/power users

The gist is that Arch pretty much comes with little to no defaults. So, you are literally put into the position to make all the important decisions. Which, as you might have imagined already, basically requires you to be pretty knowledgeable on Linux in the first place. Thus making it mostly unsuitable for new users. Though, I won't dismiss a special breed of newb that somehow manages to rawdog it quite 'successfully'.

because it has regular updates

We refer to its release cycle as rolling release. Which basically alludes to the absence of a point release.

The version of Linux Mint you were on is 21.2 and you'll soon be on 22.3. After some time, you'll be on 23.x etc. These are referred to as point releases. So, whenever a new point release update hits, you'll receive a couple of months' worth of updates. And between two consecutive point releases, you'll receive little to no updates. So, basically, Linux Mint deliberately chooses to hold updates of. By doing so, it ensures you'll only receive updates that have undergone thorough testing.

Arch, on the other hand, has a much leaner testing phase. Heck, as pointed out earlier, it doesn't even wait for a certain moment to reach before it outputs an update. Instead, after the packages have had some testing, it pushes the updates out for its users to receive it. As such, you'll receive constant updates. And you're somewhat expected to at least perform daily updates. By doing so, it ensures you'll always have access to the latest and greatest.

Note that Arch is not the only rolling release distro. But, out of the 'Big 3'^[The others being Debian and Fedora*.], it's the only one that is rolling release by default.

Its release cycle does indirectly contribute to Arch being less newbie-friendly. Basically, any update comes with the risk of causing breakage. On Debian, this risk is partly mitigated by the infrequency of updates and by pushing out very well-tested updates to begin with. On Arch, you just have to deal with it every once in a while.

Note, however, that it's most often your fault and not Arch's. Secondly, after dealing with this a couple of times, you'd have acquired some excellent skills in troubleshooting.

highly mutable

Traditional distros are basically equally mutable. So, Arch doesn't (necessarily) outdo e.g. Debian or Fedora in this regard.

It has its own repo too by the looks of it?

It does. But that's a thing with independent distros. Linux Mint is a derivative of Ubuntu. Which, itself is a derivative of Debian. Debian, however, is independent. Similarly, Arch isn't derived from anything else; hence, it's an independent distro.

The list of independent distros isn't huge or anything, but I suppose there are at least a couple of dozens of 'm.

As for the own repository part, both pkgs.org and repology.org feature some resources on that.