What’s the bestapproach this issue while shooting at sunset?

So I have a Nikon d5600 and several lenses. I love shooting portraits at sunset. With sun behind the subject, they become a silhouette. I bought an off camera flash for a fill light, but cannot use high speed sync with my camera.. so someone here recommended ND filter. This works lovely in combination with flash at max shutter speed of 1/200. However, now there is motion blur with moving subjects. Is there any way of getting around this without upgrading camera for what I’m trying to accomplish? My camera does not let me use shutter speeds over 1/200 with flash due to incompatibility (older model camera body). Thanks! You have all been so helpful.