If the person you are doing this one-step for wants long-lasting protection, then a one-step is not the method to attain that objective. You will need to apply a LSP (Last Step Product) like Collinite 845 Insulator wax (liquid) or 476S Double Wax (paste). I would prefer the 476S, but it's a little more finicky to remove than the 845, but it last longer than 845 IMO.
The Megs #16 Paste Wax is great for depth of shine/reflectivity on dark colors, but it's not the longest lasting wax.
Here's another tip: if you live near a boating/marine supply store you can probably purchase Collinite waxes over-the-counter. The 845 equivalent is sold as 925 Fiberglass Boat wax and the 476S equivalent is sold as 885 Fleet wax. How do I know? Check out the MSDS (Material safety data sheet) required by the federal government OHSA (Occupational Heath and Safety Administration) regulations for their chemical products produced and sold to the public on the Collinite web site. You'll see the 476S/885 and 845/925 on the same sheet, indicating they are the same product sold under two different labels.
How old is your tin that you have of #16? The older it gets, the more the VOC's (volatile organic compounds, which have since been banned from production and, hence, its sales) evaporate, leaving a much harder and more difficult wax to apply,especially if it's stored in a garage or shed that heats up in the summer. This is why some detailers store their (expensive) carnauba waxes in refrigerators or cool areas.
On a related note: how does D151 Reconditioning cream compare with M66 Professional Quick Detailer?? I have used their consumer-version, Color-X, as a one-step-on-steroids cleaner wax. It was much better than M04 Cleaner Wax for cleaning and correcting neglected vehicle paint surfaces, but not-so-great for durability.
