I’m kind of late to the party on this, but you mentioned you use Mouser. Mouser is my best friend when it comes to keeping BOM’s straight for each product. Will it track your inventory? No, but you can keep a nice online listing of BOM’s for each project/product. I have invested a minimal amount of money over the last ten years on various invoicing and tracking software and I found that the NCH products were the best option for me. Not too expensive, but I know you are looking for a free option, so it’s just a suggestion if you ever get to the point where you want to spend some money.
Overall, I have found a good combination of Mouser’s BOM manager, some good homegrown spreadsheet systems and my NCH software has kept me on track 99% of the time.
Good luck with your project!