Flash has to be diffused. Either bounce two identical units off a white card a few inches behind the negative at 45° angles, or put a thick Perspex or Plexiglass diffuser between the flash and an inch of airspace behind the negative.
Focusing needs to be accomplished with some sort of modeling lamp. Classic slide duplicators like my old Bowens Illumitran had built-in modeling lights for composition and focus, and flash illumination for the main exposure. BETTER units used an inverted dichroic color enlarger head (lamp house for a tungsten light source).
If I were you, I would avoid flash. It’s a more discontinuous spectrum source than better LED sources (I use a Viltrox L-116t LED video light, available on Amazon). It does require a diffuser to be added above it, at least an inch or so below the negative holder.
Since from the photo it appears you already have most everything else pretty well optimized, I’d go for an Essential Film Holder. It’s been the best money I spent on digitizing gear since Negative Lab Pro and the Viltrox LED fixture. It keeps film FLAT, and includes a proper diffuser. Negative guides are available for all sorts of formats.
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