HAF 912 case fan setup

you will get diminishing returns after the one 120mm in front, top , and rear set up. If you want to add more fans, I would start with a second fan up front followed by one up top although those second fans tend to do little as compared to the top fan over the CPU heat sink.

FYI, the CM 212+ does not see much improvement with two fans from one. The heatsink is just not big enough.

What do you think about the 200mm vs 2x120mm in front or on top?

you won't see any real world temp difference. the 200m should make less noise since it will run at less RPM to move more air then a 120mm fan. With two 120mm fans, air movement would be similar but noise may be an issue (if that is an issue with you).

really proper airflow is more important then sticking a fan in every port you find. cool air coming in the front of the case (or bottom but not both) and then leaving the top/back is the key. Adding an extra fan here or there will help alleviate hot air out of the case once the PC is no longer stressed but necessarily help with temps at load.

I have dual 140mm on top of my corsair case but my temps at load are identical if I only run a single 140mm fan. Experimenting is always the key so try different set-ups with your PC and keep track of temps at load with SW like HWmonitor.

Currently I have 1x200mm fan in the front and 1x120mm fan on the side drawing air in, and 2x120mm fans on the top and 1x120mm fan on the back blowing it out. In addition to the 2x120mm fans on the cooler. This is my first build so I'm not 100% sure what proper airflow should look like.

your airflow is set up properly

play around with your side port fan though, they can often increase GPU temps

Experiment as suggested.
I'd probably make use of the 2/200mm fans.
One in top and one in front.
Although 2x120mm would be ok in front.
Never had satisfying results with side panel fans but 1-2C temp decrease is better then nothing= never liked the extra dust the sidefan introduced