Well i run a dual boot (7,10) and a usb with linux mint at times. And frankly 7 works better in the majority of games. Sadly However microsoft studios and the games they released recently dont support windows 7.. heck they don't even support windows 10 before patch 1909!

for example - Forza Horizons 4
As such with this in mind - windows 10 is the eventual route we all have to go.
SO - your options are any of these.
- 1.dual boot - use win7 for the 98% of current games and win10 for the other 2%/. *(this is what i do and if one os has a problem i can always quickly get to my work and fix the pc later!)

- 2. Load windows 10 and use compatibility on the win7 games - and if they still dont run try a virtual machine.
About the dx12 - it gives a lil better features.like with advanced AA FXAA support. but in some games frankly on old hardware - dx9 and dx10 may work better than the 12 because those additional features sometimes leads to fps drops..
another side note looking at your gaming rig - be advised - if you have poor fps always drop SHADOWS first. (leave the rest of your settings up) amd use ur native resolution... if still bad then drop the resolution. If still not acceptable them lower the other settings.
As long as you dont fall into the "hype" - gaming at 1280×720 px or even lower is fine as long as you can interact smoothly.
Pretty graphics amd 4k resolutions but under 20 fps - wont save you from the man from the third world playing at 1024x768 on a 18' monitor with settings on minimal - who moves buttery smooth at 60fps.
