I'm building a new gaming PC for the first time in over a decade but reusing my Fractal Define R4 case and while running the power cables I noticed that my 2 front case fans are connected to this ...

Any decent fan controller in the last 5 years, probably even 10 years, is more likely to use SATA power. Of course you can get a straight molex fan adapter if you just want to run them at full blast ...