You might be able to simplify things by just going a distro like Bazzite. It's based on Fedora and has all the addons for gaming. I've got it on a laptop and it has given me fewer issues than Mint on my big gaming PC.

Linux should play well with other distros on different drives, it's only Windows that gets shitty if you try to dualboot. Whilst having a bunch of different OSes on the same machine should be fine, trying to get as much as you can out of one will just make things easier

I would just personally stick with Fedora. I know why people recommend Mint because it's easy to help people transition from Windows, but it's based on either Ubuntu or Debian which holds packages updates back for far longer for 'stability'. But if your main purpose is gaming or general desktop use or really anything that doesn't involve critical infrastructure that should go offline as minimally as possible like a server, this stability doesn't realistically help much and can actually be detrimental. For example, there have multiple optimizations in the kernel this year that have done things like improve overall performance using AMD processors by 2- 3% or lowered necessary power consumption by eliminating unnecessary processes. These updates are more rapid and effective if you use current gen hardware that has not yet been fully developed around, like a current gen GPU/CPU. Like you said, you're only really losing out on these optimizations by sticking with something that holds back packages that far. Fedora doesn't release immediately with minimal testing like Arch, so there's not usually much to worry about outside of big version updates (Fedora 48 - > 49, or KDE 5 - > 6), which I would typically wait a few weeks to upgrade to.

If you want to mainly test different desktop environments like GNOME, KDE, Cinnamon, Cosmic, etc, then I would really recommend using an atomic version of Fedora, or even better something like Bazzite/Aurora/Bluefin. Atomic refers to them only ever updating fully or not at all, so there's less chance of breakage due to a system shutdown or package conflicts. It's like flashing your phone with a stock image from Google, Samsung, or whoever and it comes with whatever apps, drivers, etc that are pre-set or chosen by you. The cool thing about it is that you can "re-base" to another version without touching your user files by entering a command and restarting your computer. There's programs like Mending Wall that help maintain any system files that may be affected by this, typically only necessary if changing between Gnome and KDE that store things in the same place. This means you can quickly switch to any other atomic variant when you feel like trying out something new without needing to reinstall your OS or try to make two conflicting desktop environments work together. Since your user files aren't touched, you can just boot Steam up in either one and your games will still be there.

I have never once before heard of someone dual-botting two different Linux distros. IMO this seems very inefficient. Personally I would just pick and stick with one.

I've had to update the Mint kernel (failing the first attempt) to get a game or a driver to work. Is there any reason to stay with Mint versus something more cutting edge?

You probably will run into the "issue" that you'll use one way more than the other, making the dual boot kinda redundant, but in principle I don't see anything wrong here. People generally overestimate the differences between distros. Both are perfectly good for both gaming and browsing; you will very rarely find a game that behaves differently on these two.

The main difference between Linux distros is the software repository. You can pretty much install any UI on any distro. You can install the Linux Mint UI on any other distro, it's called Cinnamon DE.

Do not use the Nvidia installer script. The one file that ends in .run or whatever. Chances are that it will brick your install at some point.

READ THIS. It is rules for installing software on a Debian-based system to not break your system. https://wiki.debian.org/DontBreakDebian

Nvidia hosts a software repository for Linux drivers. It says that this repository is intended for servers but it also works for desktop computers. I have the nvidia 580 driver installed on Debian. I tried the 590 driver and it was buggy.

https://docs.nvidia.com/datacenter/tesla/driver-installation-guide/debian.html

https://docs.nvidia.com/datacenter/tesla/driver-installation-guide/ubuntu.html

https://developer.download.nvidia.com/compute/cuda/repos/

Depending on which Mint you installed, you would add the Debian repository or the Ubuntu repository. I think Mint doesn't even maintain their own full repository, the Mint distro piggybacks on Debian or Ubuntu. look at your /etc/apt/sources.list

Note that, the DontBreakDebian guide says it is bad practice to add third-party repositories. This is a special circumstance.

Others have already advised on the OS side. I'm here for the gaming side.

I find protondb incredibly useful. If you don't know it, bookmark it. It's community sourced tweaks for games to get them to run on Linux when they don't just work out the box. I don't have to visit it often, but when I do it saves so much time

My plan is to give each Linux distro its own 1-terabyte SSD. So the question is: is this overall a bad idea?

Yes. It will work, but it will waste a lot of space. Individual distros are not going to require more than 200-300 GB*, and that's if you're really trying to pile everything on there. Your personal files (photos / videos / music / games) are what will take up the bulk of the space. This plan could be improved substantially by splitting one drive down the middle, installing a distro on each 500GB half (put the EFI System Partition there too), then using the other drive for a 1TB /home which is shared by both distros. This would also allow you to replace either (or both) distro while keeping your personal files and settings in place. By having your personal files on their own separate drive, you also have the option of unplugging the drive when you're fucking around with partitions and formatting filesystems, as an extra precaution to make sure you don't wipe your personal files (back-ups would be better, but it is still a nice option).

Since 500GB (each) would still leave a lot of headroom for most distros, consider partitioning swap space out of the first drive as well. At least as much as you have available RAM. Shrinking each root partition by 64GB to create 128GB of swap won't cause you to run out of space. You need at least as much swap as you have RAM to enable hibernation, but hibernation should never be used as long as two OSes are manipulating the same filesystems. Less than that just prevents the OS from killing arbitrary processes when it runs out of memory. You won't need this until one day you try running a PDF through ImageMagick and find out you actually do.

Also, Windows likes to partition TINY ESPs (100MB in my W11 virtual machine)! If you are dual-booting at all, and especially if you are dual-booting multiple Linux distros, I recommend giving the ESP some more space (like 1-2GB) so it has plenty of room for multiple kernels and / or bootloaders on it.

I am skeptical about old drivers fixing anything. It sounds like a coincidence. Not impossible, but probably the side-effect of a bug which has been fixed in one way or another. I would at the very least try other versions of WINE or Proton before to see if they work with the current driver before giving in and installing an entire extra OS to toggle back and forth between driver versions. I mean if it works, don't let me stop you, but there has to be a lower-mantainence, lower-overhead solution. It can be nice to have a second distro lying around for experimentation or rescue though. I've done this before. Knowing that something does work correctly on some other distro can be a great help in troubleshooting why it isn't working correctly on your preferred distro. Being able to cross-reference software versions, configuration, etc. This is how I learned there are in fact two separate drivers for the Sony DS4 controller in the mainline Linux kernel.

You really can't go wrong with any linux distro. They all basically work the same, with some minor differences in their respective package managers. Try several and see how they feel for you. One option is to run some of these distros inside a VM to quickly try several flavors side by side, but still keep your familiar Mint environment and web browser in case you need it. Most VM software lets you save the state (much like retro game emulators) even during the installation, allowing you to experiment with different configurations in a non-destructive way.