Download macOS Sierra (quit the App, compress it and back it up)
Download and run UniBeast to your computer specs, i.e graphics.
When UniBeast completes, restart pressing F12 - select your UEFI UniBeast drive.
At the Clover boot screen select your Yosemite drive and test booting it.
You've installed the web drivers for your 750 but the UniBeast default is nv_disable=1 so you won't have accelerated graphics unless you change the boot argument to nvda_drv=1
If you are able to boot consistently, you can begin to remove old files/kexts.
When you're happy boot your Yosemite drive with UniBeast and run the Install.app in /Applications.
When it reboots select Install macOS from your Yosemite drive.