To give some numbers, the last period of the stone age (Neolithic) lasted around 2000 years and the bronze age around 1600 years. No wonder they "forgot" what the stone age tools were.
You can set up the boot directly into Big Picture, there are a couple of ways depending on your needs/expectations.
Gamescope did not work for me, I have been gaming exclusively in Linux since proton was published but any time I try to get gamescope working it behaves strangely. I blame my Nvidia card but it's hard to say.
It became what it is currently the Steam Deck OS or at least the lessons learned were applied to create it. That being said you have distros like Bazzite and Pop OS focused on gaming, you could try those.
I recently deleted my Windows partition and went full Linux for my personal devices. I use Windows for work and it reminds me that I made the right decision.
I saw the other comments, BTRFS appears to work fine, I wouldn't use it because it is unfinished (there are some features not ready according to the status page) but I guess it is stable.
What do you mean splitting the disk? I just recently removed windows, moved and resized both boot and primary partition to take over all the space, now it is a pure Ext4 disk. I would not use BTRFS for your use case but that is up to you.
I would go with Linux Mint if you don't want to tinker with it much, but Arch will allow you to be in the bleeding edge easily if you install things from the AUR but you could stay in the stable channels as well.
Regarding games, I'm mostly a Single Player kind of guy and basically everything I have tried so far just works if it's installed from Steam, for others you can use Lutris or Heroic Launcher but tinkering might be needed.
All of this just to say: I use Arch btw.
It's been practically 2 years already since I haven't boot into Windows for gaming.
Serious response I agree with Pogo, it looks like a handmade RAM.