Linux

I like stability and cleanliness. Security by default. Least mental load possible long-term.

Excellent breakdown of your desires! FWIW, I definitely resonate with these as well.

I'm currently testing out NIXos. Next will be VanillaOS, 3rd will be Fedora Silverblue.

One simply can't ignore the fact that these are so-called atomic distros. Which makes a ton of sense considering what you set out for. FWIW, my personal takes on the individual projects are as follows:

• NixOS is pretty excellent. If the epitome of cleanliness is reached with becoming stateless, then there's simply no other viable alternative.
• For VanillaOS, I feel it has yet to fully realize its promise. Or, at least, hasn't fulfilled whatever's required to break into the (relative) 'mainstream' for one reason or another.
• Fedora Silverblue has been my daily-driver in some shape or form over the last three years 😅. As such, I'm clearly biased. However, I'd reckon secureblue, i.e. a derivative that goes all-in on security, is actually more interesting for you.

Anyone have good recommendations? Easy backups, stability, security first posture, least maintenance and memory load. I hate getting scattered in symlinks, scripts, and filesystem placing.

Honestly, with Fedora Atomic and Nixos, you're already considering the very best at the job. Though, for completeness' sake, consider looking into openSUSE Aeon as well. While I'd argue the other two are currently more interesting, I wouldn't want to dismiss it altogether.

Beyond these, we find some other distros that miss something crucial for them to be considered a legit candidate/alternative:

• Guix System can put up a decent fight against NixOS and may even sway you over if you're into lisp. Unfortunately, though, it has yet to receive what flakes brought to the table for NixOS. Don't get me wrong; Guix' implementation of channels is vastly superior over Nix' and therefore Guix System doesn't gain as much from its (to be) flake counterpart. However, with flakes, NixOS becomes pretty smooth sailing. Like, you can just trust it to work reliably. With Guix, however, it can get ugly sometimes. Which can even lead the biggest Guix proponents back to NixOS...
• Kicksecure is another hardened-by-default distro worth mentioning. Sadly, unlike secureblue, it does nothing with atomicity.

What are some pros and cons of different distros?

This is too broad of a question 😅. If possible, narrow it down to some face-offs you're particularly interested in. After which I will try to help out if I can. Btw, I 'found' this comment that attempts to assign tiers to distros in terms of how they fare security-wise.

What do you daily drive as a power user?

Without going over what a power user is and/or if I would even qualify as such, I've been daily-driving secureblue for over a year now.

Give me your thoughts and recommendations! Thanks.

At this point, I think both NixOS and secureblue pose as the most interesting candidates for ya. The former peaks in cleanliness, while the latter peaks in security.