Proton is required for running Windows games on Linux. If there is a Linux version of that game, you don't necessarily need to run Proton. Though, most developers don't seem to release the Linux version on GOG even if it's available on Steam. (Check Tomb Raider GOTY.)

Anyway, most Windows games already run really well with Proton. (I play exclusively on Linux too.) The exceptions are usually games with extensive DRM or anticheat software, but I don't think that's really a problem on GOG. If you want to check the compatibility of your games, you can also use ProtonDB.

Professional software, on the other hand, is a bit trickier but you could try using Bottles to run Windows applications. There are some pretty good alternatives so keep an eye out for those too. I'd really recommend using a spare laptop to run Linux and test it out for yourself.

Guide: https://www.youtube.com/watch?v=v9tb1gTTbJE

Clarification: Proton is just Wine with a flashy dress and extra patches. Most games don't require what it introduces, but I imagine it helps those games which do.

I'm making final preparations for converting to Linux, I think I'll go with Zorin since it looks pretty slick and was recommended by Explaining Computers.

But I've got some other questions, more hardware related than OS related.
Since my SSD's are near 5 years old and while their health is still okay, they're also small size (120 GB) and they're 3.5" SATA so not ideal read/write bandwith.

One of my PC's has an M2 slot on it's mobo so I should be able to install a new M2 SSD without issues but my backup PC hasn't got an M2 slot. Now I've come across PCI-e M2 adapters and I thought I should get better bandwith and they're pretty cheap or should I go with a SATA SSD? The reason is that the site claims there could be compatibility issues with this thing.

When any storage issue may mean data loss or corruption, I personally wouldn't risk adapters for those. But YMMV.
On the other hand, there used to be those that went into PCI-E slots directly, but now with M.2 being the norm they pretty much vanished.

I looked into PCI-e SSD's as well but those are very expensive.
I'll probably just convert one PC to Linux for now and put the other one on the waiting list to see how it goes.

Zorin OS is literally just Ubuntu with condiments in a fancy dress.

Yes, I figured that out.
Have you recommendations? Should I go with Mint?

Go with Manjaro, or Solus.

I'd sooner suggest Fedora or one of the Atomic flavors.

While I like the channel and watch it sometimes, I'd be very wary to choose a distro based on their recomendation. Not saying Zorin isn't good, just the channel use case isn't probably your use case...

Not trying to start a distro war, if you have the oportunity, download a few and try it on a pen drive. It will be a live system and any changes will be lost after shutdown but after playing a little, you will get a "feel" for whats working and what's not. Note that during the live system, any changes you do will use RAM, if you try Lutris or Bottles and install a game, will use a ton of system memory.

In normal use case, say, web browsing, video playback, playing some games, etc, you'll hardly notice a meaningful diference between a middle-of-the-road PCIe/nvme and 2.5" SATA SSD's. Even if pretty much any SSD will saturate the SATA interface (around 500MB/s) and PCIe 4.0 SSD's are usually 10 times as fast.

M2 slots normally comes will 2 interface flavors, SATA and PCIe. My AMD laptop uses PCIe/nvme (it can boot from a sata ssd but will have issues) while my intel laptop uses SATA only. Desktops usually support both modes.
Also, you should check how your backup pc uses the PCIe lanes. If you're using the PCIe 3.0 x1 (older spec, smaller size) the bandwidth is not much higher that what SATA can achieve, plus the conversion overhead...

I've never had a lot of luck with storage adapters, most used were IDE to SATA adapters. Some didn't work for long and some computers didn't like to boot from them. PCIe add-ons (such as SATA converters or USB expanders) may not be recognised on boot and you only know trying it, because it's up to the manufacturer to implement it, as far as I know. After boot never had a issue though, as long as the driver is installed.

That said, PCIe/nvme ssd's are very cheap, to be honest, for the price of a SATA adapter you can probably get a new small SSD. If you want to cheap out, I see starting from 8/9 Euros from a trusted shop in my Country, not imported.
On my intel laptop with Linux Mint installed, I have a 2 year old "Intenso" branded SATA SSD with 256GB that was around 15 Euros, it was only temporary but you know what they say...

Edit: Oh, the distro war already begun, nice....
I use mainly Mint BTW :)

The good thing is that you can change a distro later as well, if you feel certain distro is not so nice to use.
Naturally it usually means reinstalling Linux from scratch but pfft, just copy the stuff you want to keep in your /home folder somewhere and then wipe everything out and start a new installation...

Linux Mint is an ok choice and you will find plenty of instructions online how to use it (ie. to get games work on it), especially as Ubuntu instructions mostly work for Mint too.

BTW, when someone recommends a certain Linux distro for you, remember to ask why you should choose it over other distros. That way you can make up your own mind which features or reasons are important to you.

For a Linux newbie Mint (22.1) is the go-to distro, but you can use it even if experienced. It looks similar to "windblows", easy to use, has a GPU driver manager, good software center (that's how you install most things, don't randomly downlaod stuff off the internets) with flatpak support. For gaming i'd go with the XFCE version. XFCE is lightweight, fast, customizable, you can completely disable the compositor and prevent potential micro-stutters in some games, lower the input lag and comes with the best(?) file manager, Thunar. You can test whichever version or even distro you want in a live environment. Write the image to an usb stick using rufus and boot into it.

Another solid choice is EndeavourOS, "Arch for (normal...-ish) people", It's a rolling release distro, meaning that you get the latest stuff pretty much as soon as it's available. Latest kernel, drivers, the works. You'll have to use the terminal with this one at least for a bit after installation. Scary stuff, i know. The rolling release nature of it makes it the best suited for new hardware (runs well on old hardware too) and if you want to play, or have have the best chance to run the newest games.

If you desire a Steam Deck, or console-like experience the best one at the moment is Bazzite, imo.

For GPU tweaking you can use LACT or CoreCtrl.

To play "windblows" games outside of steam, in their own "container" you can use Lutris, or Heroic Games Launcher... or Bottles. DXVK_FRAME_RATE=40 (whatever framerate you want) - command to limit FPS when DXVK is used; another way to "tweak the GPU", limit power draw, if the game doesn't have such settings.

linux mint has a debian edition, that one has nothing to do with unbuntu.

By the way: I choose Fedora because I'd go daffy waiting for updates on Debian derivatives with their fixed point releases, and yet I'm not so gung ho for new things that I am actually willing to wait for a testing period. (I say this, on Fedora 42b.)