Congrats, looks very good. I actually also re-did the seats in my E32 750 with similar leather care from a shop in Germany. The liquid leather or as you call it crack filler works nice and it makes fun to do the work.
I even repaired a sports seat on my friends Alpina B12 where the side bolster was completely torn. I remove the complete leather from the seat, used special kleather glue and glued a leather patch from the back side onto the leather, then I turned it again and filled the mating areas between the patch and the original leather with crack filler so that it got smooth and the final product looks very good.
Besides leather refresher in black I also bought this special leather paint as concentrate to dye the patch in black so that it matched, the leather refresher color is usually 'stretched' and not that concentrated.