What you wrote earlier wasn’t entirely clear. I suppose I filled in the gaps with a common scenario where some play their Steam games (in Linux) but from a directory on the Windows disk (or partition).
If you don’t do that; if you only play the games in Windows; then you can safely ignore my comments regarding Steam. 
As Manjaro and Windows are already on completely separate disks (and already functioning well) most of those concerns are not related to your scenario.
The worst that might happen is if Windows somehow resets Secure Boot, in which case, boot to BIOS and make sure it’s disabled again. Otherwise, following the procedure in my previous post should help avoid most potential issues.