Problem with decreasing polygon count.

If I may ask why you are converting high poly model to a low poly one? What is main the main goal? Screenshot will be more helpful.
Also try to avoid custom normals, I got a lot of headache with customers because not every software calculate those on import as it should.

Hi! I am making game ready models since years, so I have some experience. You can't automatize your lowpoly workflow with modifiers. Most of the cases I modelling the low and the high poly at the same time part after part, and after all the part are finished I start merging them together. When I created my Old home computer, the first part what I make is the keyboard, first I made a high poly version with separated keys, and after that I made a very very lowpoly version of them. But watch my home computers last two pic: https://www.cgtrader.com/3d-models/electronics/computer/old-home-computer-microcomputer
and you will understand the logic. If you saw the pic you can understand there are only a few polygons for many many keys, and the details come from the normal map. Sorry for my English!

I don't work in the game industry but decimates hurt me a good retopology for a gun doesn't take a lot of time and you can modify it easely after if you need to. Buyers will always be happy to have a good topology.