How to make best milk chocolate at home?

To really be considered chocolate, you need to use cocoa butter as your fat. Cocoa butter has a few properties that other fats don't have. Most notably, it is capable of forming a crystalline structure which is what gives tempered chocolate its "snap" and—when tempered—it has a melting point above external body temperature but below internal body temperature. Using any other fat will result in a chocolate flavored material that is not chocolate (more akin to frosting or fudge).

The other factor in the "expensive" chocolates at the store is the particle size of the cocoa powder. This can be in the neighborhood of 20 microns in a good chocolate, but can be half that size in high-end chocolates. To achieve this, the chocolate needs to be physically worked for many hours, if not days. Commercial suppliers use a machine called a conche to accomplish this. Cocoa powders usually contain particles in the 35-50 micron range, so you'll never get that "expensive chocolate" mouth feel.

In addition, the temperatures, humidity, roasting times, mix of beans, type of sugar and fat content of the milk powder are all precisely maintained, further impacting the flavor.

You may have trouble with milk powder at the store because it is usually made with little or no fat to extend shelf life. The same goes for the fat content of cocoa powder, as there is a wide range in the available products. Check to see if your recipe specified a fat content for the milk powder and cocoa powder, and if not, you might need to experiment.

As you said, you can make "pretty good" milk chocolate at home. The most difficult part will be tempering the chocolate, but it's so common to need to re-temper commercial chocolate when you work with them that it's a good skill to master anyway.

Most of the people I know who make chocolate at home do so because the vast majority of chocolate makers use soy lecithin as an emulsifying agent and they (or their loved ones) can't eat soy, but even then, there are a few manufacturers who make soy-free chocolates these days.