Can't open files from Adobe Photoshop

Why they carried that date (2017) forward until early 2019 I don't know.

That's standard Apple stuff. If a model is released in 2017, and the hardware doesn't change (other than minor point revisions), it's always designated by the year that model was first designed, not the year is was built.

What does change, even though entirely unnecessary, is the firmware. Had you bought that same Mac in 2017, it would have shipped with Sierra. But (I would guess) for no more a reason than to make a Mac built two years later seem like a better or newer version, they change the firmware to only allow the current OS or newer to be installed, even though Sierra would still work just fine. That's how you ended up with Mojave.

Creating a new partition:

1. Can't stress this enough. Have a full, restorable backup on hand FIRST. Just in case anything goes south, you can at least restore the Mac to the point it was at before you started.
2. Open Disk Utility. Highlight the drive name itself at the top in the left column. Click Partition in the top row.
3. 
4. Assuming you only have Catalina installed and no other partitions, you should see something like this. Except it won't be divided into three parts as my drive is.
5. 
6. Click the + button to add a new, hard partition. It will default to splitting the space, but at this point, you can make the new partition any size you want. Either by moving the dot on the wheel, or just entering number of GB you want (where mine says 150). You're kind of limited with a 250 GB drive, so be careful how you split it. I have Mojave on a 55 GB partition. The OS and most of the CS6 suite is on it. Even then, it still has 27 GB of free space. So you don't need a ton of room. Just make sure not to make it too small. A drive that's almost full is always bad!
7. Click Apply when you have things sized the way you want them. It should only take about 15-20 seconds to complete. Quit Disk Utility.
8. Restart the Mac and immediately hold down the Command+Option+R keys. Adding the Option key is supposed to tell the Mac to install the OS the Mac shipped with, not the same as what's currently on the drive.
9. When the work screen appears, choose to install macOS. Check to make sure it asks where to install Mojave. It may want to overwrite Catalina, so also make sure to click the button to choose another drive, and select your new, blank partition.

Rather than splitting your boot drive into two volumes get an external SSD and boot from it. I can boot from a USB 3.1 SSD in this configuration in about 50-60 seconds.

Booting from my internal SSD takes about 25-30 seconds. So there's not a big difference. Once booted speed seems the same. I have several SSDs I use for different systems and external storage.

OR you can get an external SSD in it's own case if you prefer.

Sorry, no. I can only guess it's a limitation of trying to use unsupported software in Catalina. Even in Mojave, After Effects CS6 will not launch.

Why it worked at all for the previous few months is a bit of a mystery. It could be a recent update to Catalina is the cause. Such as 10.15.2 to 10.15.3. Strictly a guess though on that one.

I'm fortunate enough to have the CS6 Master Collection. Mainly because there is no affordable, full featured DVD/Blu-ray authoring software anywhere.

I bought a 2018 Mac Mini that still came with Mojave so I could install it and CS6 on a separate partition. According to Adobe, it's still legal to use Encore if you already have a license. So I boot into Mojave whenever I need to do any disc authoring.

It would first depend on what your 5K iMac shipped with. If this is a very recent model and it came with Catalina installed, there is no way to install an older OS. This is normal for the Mac as an older OS doesn't have the drivers for newer hardware.

Both are interesting ideas, my concern with the dual partition is that since I only have 256 GB to play with, performance might take a hit?

If you split your 256 GB internal, performance will be identical. Any external SSD will be a bit slower, but it isn't much different as long as the connection is USB 3 or USB C.

I took an SSD out of an older Mac Pro I sold and put it in a USB 3 housing. Other than the boot time is a bit slower, once up and running, you can't tell much of a difference. The item Old Toad mentioned is essentially the same idea, with the only downside (slight) is the drive isn't protected from anything that may hit it.

I use a lot of fonts... do they go on the root drive or can they be on an external drive?

Doesn't matter. Unless you're talking about a massive number of 'em, fonts don't take up much room. I have a folder with about 29,000 fonts in it (under various subfolders) and all of that only takes up 12 GB of space. You can put them pretty much anywhere you want, other than the System folder.

I would say don't drop them all into a Fonts folder. Fonts take up space in RAM, lots of them being active will naturally eat up a lot of RAM and cause your apps to drag as they struggle to show mile long lists. Use your font manager to activate what you need as you need them.

And I think from what Kurt is saying is that if I repartition, Mojave is buried someplace in the hardware (maybe) and it'll install it without too much of a hassle.

No, it isn't anywhere on your Mac. That's just what a Command+Option+R start should install, which will all be downloaded via the Internet from Apple's servers.

Toads idea also has its merits.

Indeed it does. An external SSD on a USB 3 or USB C connection is plenty fast.

The CS6 suite does not work in Catalina. What version of PS are you using?

Please note!

I put down the wrong keystroke. It's Command+Option+Shift+R to install the OS your Mac shipped with.