So, you can indeed replace the HD in the iMac with an SSD, but it's not for the faint of heart (a 2015 MBP is a piece of cake compared to this thing!) Given the situation you describe, I'm not sure I'd be particularly eager to try it. See:
https://www.ifixit.com/Guide/iMac+Intel+27-Inch+EMC+2546+Hard+Drive+Replacement/15796First question:
1. Do you have a COMPLETE (ie: cloned) back up on your existing iMac internal hard drive? You should have at least one backup (preferably 3 - one TimeMachine, one clone on premises and one clone somewhere else - this is known as the 1-2-3 rule). But at the very least you should have ONE.
Your Mac is likely still slow because it's only got 15% free. Also, the internal drive may not be in the best of shape 7 years down the line. DriveDX is a worthwhile investment, in my books, to keep an eye on your drives (especially if you have as many as I have, which is well, a bit crazy, but so it goes). I would try moving the majority of your files off to another drive - if you're not actively using them, then it's not like you'd be creating a bottleneck because of slow USB speeds etc. But you must back up THAT drive as well. You can keep the active copies of files you're actually working on, on the old iMac.
Your Adobe 6 files will all be openable in either current versions of Creative Cloud (which I refused to pay for as well, as it's crazy expensive) OR the Affinity apps. I do subscribe to the Photography CC package - with 140,000 photos in Lightroom it's just too mind-boggling to consider something else, but it's relatively reasonable at $20/mo CDN. However, I use Affinity Publisher for all desktop publishing work and it does now open InDesign files natively. So you would KEEP the original Adobe files, but open them in Affinity on your new iMac (and save them as Affinity files going forward). It's very capable - I know there may be a few things it doesn't match Adobe on, but I've not had any issues with what I'm doing (newsletters for a couple of organizations, mostly).
You can also open/import AI files in Affinity Designer - being mindful of a few issues - one of which is described here:
https://forum.affinity.serif.com/index.php?/topic/218985-open-illustrator-ai-files-in-affinity-designer-2-with-objectsvectors-outside-the-canvasartboards/ I never learned Illustrator, so can't advise on that - other than to read whatever you can get your hands on if you have particular concerns.
Affinity Photo is pretty good as well - though since I have Photoshop I just use that most of the time. The exception is when I'm at the historical society - we have Affinity there (no PS). It's much easier to find alternatives to PS than to the other Adobe apps.
I am very impressed with Affinity though - have been using their apps since they started. And you cannot beat the price.
Other random notes: I have a 2TB SSD in my iMac and it's not big enough for my photos - they're all on a 2TB external SSD, which is backed up in triplicate. Because my iMac is newer (not Apple Silicon, however) and has USB 3.0 it's a reasonable proposition to keep a Lightroom library on an external drive - it's not ridiculously slow, which I suspect it might be on an older iMac with USB 2.0.
Apple still makes one pay dearly for internal storage upgrades (+$750 CDN for 2TB SSD on any of their offerings) which annoys me no end, especially when I can buy an external SSD for a third of that. I am guessing that when it next comes time to upgrade, I'll be looking at a Mac Mini with a Studio Display, since 27" of 5K screen real estate is pretty much a minimum for me now, and they no longer make the 27" iMac.
