I used black wood filler and dye on my build. I really liked the results so look forward to seeing the results of yours. I was not looking for a super smooth finish so I only did the grain fill once. If I had done a couple of fills it would be easier to get a flat surface. This seems to be what more experienced builder recommend if you are going for that look.
I have been using wipe on poly for the first time and can recommend as a great way to get a good cover. I don't have a great set up to spray when it is winter so I was looking for an alternative. I did do two very light coats of the Cabothane Poly Gloss before the wipe on as sealer as I had seen it as a recommendation to stop the dye bleeding into the wipe on. Not sure if this is necessary but it made sense to me and worked.