Veraxis

1). It matters a little bit, but not a ton. A big difference will come down to what package manager comes with a distro, and whether it is a "rolling" distro that gets updates as soon as they come out, or one which withholds updates until after they have some time for bug testing, etc. Given that gaming is one of your primary use cases, I would recommend a rolling distro. If your distro does not come prepackaged with something you can usually install it. Minimalist distros like Arch come with almost nothing pre-installed, not even a desktop environment, so you can be very granular about what gets installed and keep you system lean.

2). Most distros will not have as much in the way of corporate control/privacy concerns. A few like Ubuntu or Fedora are more closely linked with a single company, but most are more community driven and this is not a concern.

3). The advice I always give to people first trying Linux is to go into it with the mindset that you are learning a new skill, as many things are simply done differently to Windows. Most things work fine, but every once in a while, especially when setting something up for the first time, it may require additional configuration steps. Very popular distros like Mint will usually have the most community resources, and you can often find posts or guides of people who have already solved some issue you run into. I would also throw one of the Arch-based distros onto your list: Endeavour OS, which is essentially a pre-configured Arch installation. The Arch wiki is one of the most highly regarded resources in the entire linux community, and even if you are not using an Arch distro, many of the guides on it can be helpful.

4). Generally speaking, my multi-monitor usage with KDE has been seamless. No issues that I can think of. HDR support has only very recently been added, so I am not sure how well it works, but it is improving rapidly.

5). I think Plasma would be a good fit for what you are trying to do. Honestly, it is very customizable, but perfectly usable right out of the box even if you do not want to do anything to it. The layout is very familiar for a Windows user.

I think it is not possible to avoid it in all cases, but the reputation and business practices of the controlling company are your best indicator. Any changes to a company's culture may give signs if a piece of software may start to employ anti-consumer tactics.

Naturally, being closed source and in a dominant market position (i.e. a monopoly or near-monopoly) would make it easy for a company to start pulling these kinds of tactics. Sometimes even formerly reputable companies with open source software can try to do things like this after buyouts, changes in management, pressure from capital investors to increase profits, etc.

Generally, open source programs will be harder to monetize than closed source programs, as someone can fork the code and take out the disliked features. See Ungoogled chromium vs Google Chrome, VSCodium vs VSCode, Rocky Linux vs RHEL, etc.

Manjaro was my first distro. I used a number of AUR packages and ran into excessive dependency issues due to Manjaro's packages being held back and often a version or two behind. This eventually led me to switch to vanilla arch. Unless one plans on not using any AUR packages at all, I do not recommend it.

Manjaro also uses the pamac gui package manager, which has a bit of a history of "DDoS"-ing the AUR with excessive requests. Apparently, the search field in pamac would begin querying the AUR after every letter typed to try and populate autocomplete results, hammering it with requests. Pamac also does not distinguish between package repos, so even just having AUR enabled and searching for a regular repo package would send requests to the AUR. Apparently it got so bad that it took down the AUR and they started returning 403 to requests from pamac users. In fact, this happened a second time and got them blocked again. This got the Manjaro devs in bad graces with a number of Linux folks as it was not a bug, but a poor design choice.

Stick to the many guides available and you will be fine. One thing which I either missed or was glossed over in most guides is to install the Linux-firmware package. It is considered an "optional" package, but on all the machines I have ever used I have run into issues without it.

If it has an ethernet port (or perhaps a USB to ethernet dongle), maybe a PiHole DNS using Debian or the like? It is apparently supported on other Linux distros than Raspbian.

If it supports micro SD XC (i.e. capacities higher than 32GB) or you have a USB hard drive or high capacity USB flash drive, maybe a samba server for file storage? I often use my file server as a substitute for digging out a flash drive any time I want to quickly pass a file between two machines in my house.

Do you have the bolt and plasma-thunderbolt packages installed? I believe I had to do that to use a thunderbolt SSD enclosure/docks with my laptop. I am not sure about DP alt mode, though.

I think the voltage boosting microcode bug on 13th and 14th gen are generally considered to have been solved. I have not heard of any issues on the Intel 100/200 series CPUs. I would definitely recommend updating the BIOS on any 13th or 14th gen laptops, though.

I think it is the reverse these days. AMD generally has better power efficiency than Intel.

How cheap is cheap? Truly low-end laptops will generally be worse value than buying used, and low end laptops tend to suffer on some of the more subtle quality of life features with poor-quality screens, cheap trackpads, small batteries, and plastic build quality.

I do not know what others think, but in my experience Asus laptops do not have a good reputation for reliability. Almost everyone I know who has owned one has had them die prematurely, even the relatively high end ones. They are also notorious for having terrible warranty support and customer support.

At least where I live, the used market on places like ebay tend to be flooded with surplus business laptops from the likes of Dell and HP which are only a couple generations old. There are also good deals to be had on refurbished/open box laptops. My most recent laptop is a refurb 16" HP Pavilion plus with a 13500H and generally good screen/battery/build quality for around $450 USD.

I cannot speak to compatibility/optimization differences between Intel and AMD, but AMD is also generally known for better power efficiency, while based on my looking at benchmarks, Intel tends to lead on single-core performance. I think both are viable. I think I would recommend getting whichever you can find the best deals on in your local market and maybe use other features you care about to narrow down the choices before deciding which brand of CPU you want.

Looking at CPU benchmarks like passmark can be a good reference. For example, an Intel i5-1235U and and Intel i7-1255U have the same core count with only a minor clock speed difference, so they benchmark to within single-digit percentages of one another, but the i5 will sell for significantly cheaper on the used market simply because it is called "i5" and not "i7". Meanwhile an i3-1215U has a lower core count and significantly worse performance, and so should probably be avoided.

Arch. I need the AUR for certain applications, and the high degree of customizability and opportunity for learning appeal to me as a relatively new-ish Linux user (going on a few years now, most of that time having been on Arch).

For system files/configuration on my machines, timeshift set to run once a week.

For family photos and shared files, I built a pair of SFTP servers made from old HP thin-client PCs at two different geographic locations which automatically sync to each other once a day via cron job using vsftpd and lftp. Each one has both an NVMe and SATA SSD which run in a software RAID 1 configuration.

For any other files, a second local server also using vsftpd and two SSDs in USB enclosures. I manually back them up using rsync on an irregular basis.

I would say a used Dell or HP business laptop would be a safe bet. Most business laptops have decent keyboards, replacement batteries will be relatively easy to find, and user-serviceable RAM is the norm. Given the not especially high processing power needs, probably the middle-specced ones with a few gens-old i5 will be dirt cheap and work fine for your needs.