More RAM would probably be easiest bump it to 16GB and that'll probably be noticeable.
The SSD seems more involved.
https://www.ifixit.com/Guide/iMac+In...placement/7555
The ssd replacement would be a huge improvement.
The video card would be even more involved as it would involve remounting the heat sink and the iMacs are more like a laptop than a desktop, doesn't look simple anyway. Any MXM style video card should work but I'd find an example of someone else doing it successfully since it'd depend on the OS having the right driver built in and that the card wouldn't overload the built in heatsink and fan. And it would only speed up games or maybe primere if that utilizes the video card. I'd be surprised if you could get a huge bump in the video card performance, might not be worth the hassle.