Mode Difference, Exclusion, Subtract, etc... in this section are good helps to separate a BG, you did well, now you just need to use that "visible" as a layer mask 
With the subtract mode on top, all the BG black, and that top layer selected in the layer's stack/dialog, right click on it > a context menu opens, select "New from Visible", select that "Visible" layer, make it B&W (Colors > Desaturate > Desaturate), increase contrast to better separate black from the subject, may be paint a bit some white on the subject to fine tune, and then use it as a mask on the needed layer.
With the subtract mode on top, all the BG black, and that top layer selected in the layer's stack/dialog, right click on it > a context menu opens, select "New from Visible", select that "Visible" layer, make it B&W (Colors > Desaturate > Desaturate), increase contrast to better separate black from the subject, may be paint a bit some white on the subject to fine tune, and then use it as a mask on the needed layer.