Upgrade Memory and HDD in late 2013 21.5 inch iMac

A few weeks ago I decided to install macOS Sierra on my 21.5 inch iMac late 2013 model. Ever since I have noticed the machine gets really sluggish after a few hours of using it. The machine worked fine under Yosemite and El Capitan. I know some users have had issues with it so maybe I am too.

I guess my biggest question is would upgrading the memory or putting in an SSD help solve these issues? Right now I have a 1TB standard HDD and 8GB of memory, so there is a little room for improvement.

My other question is has anyone with a 2013 21.5 inch iMac done these upgrades? How hard is it really? I have seen tutorials on this and while it doesn't look too bad I am still on the fence about whether I want to do it myself. At least I still have a physical hard drive and non-soldered in memory that I can swap, even if it isn't that easy to get to.

Is there something I can do without doing these upgrades that might help with the sluggish performance? I really like this computer and hope to continue using it for the next few years. I purchased it used from someone that bought it but never really used it. I am sure it still has plenty of life in it and would hate to see it go to waste.