I’m not a security expert by any means, but here are a few things I know as a regular user:
Always keep your system up-to-date and only download and execute software from the official Arch repository if you can help it. Malware often takes advantage of outdated systems that don’t have the latest security patches, so by staying as up-to-date as possible you’re making yourself a very difficult target. The AUR is a user-based repository and is not inherently trusted/maintained like the official Arch repos, so be careful and always read PKGBUILDs before you use AUR software. Don’t use AUR auto-updaters unless you’re reading the PKGBUILD changes every time. Ideally try not to use the AUR at all if you can help it; official Arch Linux is usually quite stable, but AUR software is often responsible for a lot of the “breakages” people tend to get with Arch. If you have to run sketchy software, use a virtual machine for it, as a 0-day VM escape is almost certainly not going to happen with any sort of malware you’d run into. ClamAV or VirusTotal may also help you scan specific files that you’re wary of, but I wouldn’t trust that a file is clean just because it passes an AV check. Also, never run anything as root unless you have a very specific reason, and even then try to use sudo instead of elevating to a full root shell.
Don’t open up any network ports on your system unless you absolutely have to, and if you’re opening an SSH port, make sure that it: isn’t the default port number, requires a keyfile for login, root cannot be logged into directly, and authentication attempts are limited to a low number. If you’re opening ports for other services, try to use Docker/Podman containers with minimal access to your system resources and not running in root mode. Also consider using something like CrowdSec or fail2ban for blocking bots crawling ports.
As far as finding out if you’re infected, I’m not sure if there’s a great way to know unless they immediately encrypt all your stuff and demand crypto. Malware could also come in the form of silent keyloggers (which you’d only find out about after you start getting your accounts hacked) or cryptocurrency miners/botnets (which probably attempt to hide their CPU/GPU usage while you’re actively using your computer). At the very least, you’re not likely to be hit by a sophisticated 0-day, so whatever malware you get on your computer probably wants something direct and uncomplicated from you.
Setting up a backup solution to a NAS running e.g. ZFS can help with preventing malware from pwning your important data, as a filesystem like ZFS can rollback its snapshots and just unencrypt the data again (even if it’s encrypted directly on the NAS). 2FA’ing your accounts (especially important ones like email) is a good way to prevent keyloggers from being able to repeat your username+password into a service and get access. Setting up a resource monitoring daemon can probably help you find out if you’re leaking resources to some kind of crypto miner, though I don’t have specific recommendations as I haven’t done this before.
In the case of what to do once you’re pwned, IMO the only real solution is to salvage and verify your data, wipe everything down, and reinstall. There’s no guarantee that the malware isn’t continually hiding itself somewhere, so trying to remove it yourself is probably not going to solve anything. If you follow all the above precautions and still get pwned, I’m fairly sure the malware will be news somewhere, and security experts may already be studying the malware’s behavior and giving tips on what to do as a resolution.
I don’t have any experience with iCloud Private Relay, but I’d be surprised if enabling it will make you un-fingerprintable (in which case what are you really trying to accomplish by using it?). Also, who are you trying to stay private from? Do you personally believe that Apple and/or Cloudflare aren’t selling or trading your data? Would you be okay with them being the only ones that control your data if they’re not selling it? It’s a nuanced topic, and likely you’re the only one that can answer your position on that. It’s cliché, but defining a threat model can help a lot with deciding how many conveniences you are okay with giving up. I would likely argue that an Android phone with LineageOS can be made more private than an iPhone, but at the cost of security. Does your threat model need to sacrifice privacy for security?
Regarding iPhone vs Android, I’ve only ever used Android, but my friends with iPhones and Macs never seem to have access to the open-source software that I use and recommend, so I feel like that’s a big part to consider also. You’ll get roped into a proprietary ecosystem where it seems like every little app is trying to charge you money and won’t show you what it’s doing behind the scenes. If you already have an iPhone I’d understand if you need to weigh the economic feasibility of buying an entire new phone just for privacy as well.
Personally, I don’t really trust anything unless I’m given infallible reason to trust it, e.g. cryptographic proofs, audits, zero-trust models etc.; in this world it seems inevitable that someone will take advantage of your trust either today or tomorrow. If someone is truly on your side, they will do everything they can to take the need to trust them out of the equation, and failing that they should make it as clear as they can what trust is still mandatory and why. If you want to trust someone that doesn’t meet these standards, you do so basically at your own risk, and you’ll have to start doing some mental calculus on what they could get from you, what they might want it for, and how eager you think they would be to start misusing it (e.g., if you pay for a service, the servicer may feel less compelled to subsidize their income by selling your data).