Hi! I would try turning on "Use Full Precision UVs" for the mesh in the static mesh editor in Unreal. By default static meshes use half precision UVs to save memory. Edit: Unrelated, if you re-use UV space for mirrored/repeating elements you can increase texel density, meaning less jagged edges. Could use a second UV…
Some UV editing tools allow you to convert the UV into mesh space, where you can edit with the full set of mesh editing tools. Using this, you could try a Push or Relax style tool, which would scale the elements away from each other. Then convert back out of mesh space into UV space again.