You probably should post a picture of your relationships window if you have created relationships. The fact that you mention Excel and refer to Union queries as a means of joining tables makes me think you have gotten off on the wrong path when it comes to design. You should also state how important it is to have sequential numbers, and maybe more about the business case. I'm also wondering why you chose such a number design as 100589 as a product number since that won't tell you anything about it at all. Maybe that's OK for you.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.