Confused by Freesync/fps options

Hello, having just bought a new pc and monitor I'm on catchup trying to get up to speed with in-game settings. I have a 144htz Freesync monitor and an AMD GPU and CPU. My question is, is Freesync only a factor if I'm getting more than 144 FPS? For example playing TWW2 I'm getting about 50fps, so does that mean in options VSync should be on or off? I'm a little confused.

Also, my card supports DX12 (I think?), should I be selecting DX12 in options if that's still beta? In not sure what that actually does with regard to performance.
Cheers.

AMD Ryzen 5 3600 Six Core CPU
ASUS® TUF X470-PLUS GAMING
16GB Corsair VENGEANCE DDR4 3200MHz
8GB AMD RADEON™ RX 580
AOC G2590PX 24.5" 144Hz Monitor
 
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