Why do you think you need more RAM? I have 16 GB RAM in a 2017 27 " i7 iMac and the only time I've had memory problems is when I tried to perform blur and smudge edits on 500 MB image file in Photoshop Elements.
Additional RAM would have let the blur edit go faster but it won't speed up boot time, app or file opening time. Those you've already taken care of with the SSD.
You'r model iMac is considered obsolete by Apple and anything you put into it will be worth more than its value. I would not up the RAM to 32 G and start saving up for a new Mac.
Also be aware that your model iMac can run Catalina if you're not already doing so. But if you're not on Catalina 32 bit apps will not run on Catalina. You'll need to get 64 bit updates to those apps or alternatives. The free app GO64 will tell you which of your apps will run or not.