my experience with oil paintings is, that there are a lot of reflections caused either by brushstroke or by varish. Thus I recomment constant light, no flash.
My propsal therefore is, using 4 lamps with reflectors/LED boards and put the in position in an angle below 45 degrees to he surface of the picture.
Then the main reflection should bypass the lens. Only small reflections caused by surface structure, especially in dark areas will be see.
Try around this value by experimenting. In your case, I see the problem of metering light.
For the metering I propose to use a "grey card" in the size of the painings and align the lamps, by measuring onto several spots of the card to get even light over the total surface.
Then do not move the lamps and put the picture into place.
I expect at those pictures a very strong contrast, which might it make difficult to arrange the lamps by metering onto the painting only.