I kinda feel like you're not getting as many results because it's not really as complicated in Linux. In Linux, I feel like if it's a good idea to be able to install an item wherever you want, the install process will usually have a step where you get to choose where to put it.

If you're installing your games through Steam, there's also a Storage tab in your Settings that will allow you to create new libraries wherever and then you can just choose whichever one you like to install items to. I think moving your home drive might accomplish most of everything else you want.

I had a very similar setup at one point, and what I did was just install Linux on the SSD. I'd recommend against mounting the HDD as /home as it'll make your file manager feel sluggish, and would be an overall bad experience.

I used to just symlink directories from the HDD as ~/Pictures, ~/Music etc. and most software would just be able to work with those. You don't really need to move software to your HDD since they rarely take much space, and moving to HDD will make them very slow.

For games, Steam and Lutris will let you select the installation directory on a per-item basis so you should be just fine.

That said, maybe you could use this opportunity to get a new larger SSD and then you won't have to worry about this anymore. Also, you'll be able to keep your important data on both drives, so you'll have sort of a backup when something fails. (A more robust backup solution will be better, but hey it's a start.)

I configured my previous PC that way. I set it up that way during installation. I put /home on the HDD, and everything else on the SSD. This meant that files just go to the right drive by default.

I did have to create a folder on the SSD and change the permissions so steam could put games there. Steam does not have root admin access.

This is an advanced configuration, but you can manage it. Since its during installation, you're wiping the disks so there's little risk in trying it again if you don't get it right on the first try. Linux Mint's installer has the tools and instructions on how to do this.

The directory structure on Linux works very differently from windows. Things can be linked and everything is a file, and the location of something in the directory structure doesn't match the hardware its on.

/ is the root directory of the system. /home is where all your files are stored. Select "something else" during the Mint installation and manually set up partitions, to put / on the SSD, and /home on the HDD and it should just work.

I can only speak for myself on how I'd do this, but I'd dedicate the non-SSD for /home which avoids most of your headaches. For games on your SSD drive you could just make a dedicated /games folder on your root partition.

Personally I'm lucky as I have a 2 TB main SSD (NVMe), so I don't have to choose. And then 2x4TB for storage, backups, linux ISOs etc.
Obviously you don't want "just buy a bigger SSD" answer, but I'd definitely consider saving up for that.

I think one of the issues you might be running into is that a lot of linux guides use terminal commands instead of GUI tools, because it's a lot easier to make a good guide with just terminal commands - the steps are short, you don't need large images and an image editing program for every step, and they're less ambiguous than a guide that goes "open this, click here, open that menu, then click there ...". This isn't actually that complicated once you get the hang of it (at least not for the type of thing you want to do), but it's definitely intimidating for a new user.

Games and documents should be fairly easy, chances are good that you can connect any drive you already have and it will immediately work. Software is a little more complicated, I'd honestly recommend not bothering with that until you're more familiar with Linux. The issue here is that almost all software you'll use on Linux is distributed via package managers (what's called "app stores" on commercial OSs) and wants to be installed to certain standard locations - you can move those locations to external drives, but you'll definitely need terminal commands to achieve that, if only because I guarantee that's there's no guide that won't use the terminal.

On the plus side, IME software (excluding games obviously) takes up less space on Linux than on Windows. 128GB is probably enough for all your software.

I would put everything I can on the larger drive, and install Mint on the smaller one.

I can always move files to the smaller boot drive later, if I find a reason to.

A question for others before I write a more thorough reply:

Wouldn't it be a good solution to move the directories elsewhere but then symlink them to their default locations? You'll need to think a bit about file permissions while doing that, but are there any other caveats?

For the asker:
UNIX-like systems such as Linux are intentionally made so that a directory can be on a different hard drive than its parent directory. So, you can have /usr/games, including almost everything on it to be on hard drive #1, but then define that /usr/games/quake should be on hard drive #2. The limitation is, you cannot have different parts of the directory tree on the same hard drive. So, if /usr/games/quake is on its own partition, you cannot have /home/plzgivehugs/artwork/bigfiles on the same partition.

...except, with symlinks you kind of can.