There's a slider to apply a global scale multiplier in the System Settings under "Display & Monitor". So, if you set it to 200%, everything will be twice as big.

As for making a distro gaming-ready, honestly I think that's a bit overpronounced on the webpages of Bazzite and Garuda. It's one of their distinguishing features, so that's what they'll talk about, but I'd be surprised if we're talking 5 FPS more compared to a general purpose distro.
They generally use the same software and both of them are tuned for performance, with only a slightly different focus when they'll perform the most optimal.

Yeah, I don't know what concrete difference zstd makes. The Arch Wiki (great resource, generally applicable independent of distro) tells me that compression may speed up some workloads while slowing down others: https://wiki.archlinux.org/title/Btrfs#Compression
Maybe Garuda found out that it mostly helps with gaming when openSUSE decided to not make use of it, because openSUSE is more general-purpose.

But yeah, I don't know, if you're feeling Garuda, then go for it. At this point, you could tell me that you merely like the theme of Garuda better and I'd support that decision, because what I've read about it does sound reasonable, and it sounds like you'll be fine either way.

And on linux, i should be able to edit custom shortcuts, macros and stuff, right?

Not entirely sure what you mean by macros, but: Yes.
The whole OS is built from the ground up to be scriptable and configurable. It's very likely better than you can imagine.