The ideal way to figure out settings would be to find out what the case of your device is made of, and obtain some of that as raw material to play with.
Since there is likely some coating or paint, this isn’t flawless.
Getting an extra case is likely not a cheap option.
But making friends with a local pawn shop or repair shop, where people like to discard of broken old devices, would pay off greatly.
This strategy works wonders for older devices. And in many cases the newer devices are upgrades of older ones, maintaining the same type of case. So it can work on newer devices as well.
Barring any of that at all being possible, it never hurts to write to a company and request a sample, explaining that you intend to customize and sell them. In some cases, the company just gives out samples without bothering to verify vendor status, accepting whatever random losses may happen as a lower cost than vetting every request. In other cases, they may ask for a vendor license, or an estimate of sales level anticipated.
If you aren’t successful in THAT endeavor… then you start practicing on your own device. Line up what you want as a final engraving, and run it at the highest speed and lowest power. Likely this does not engrave at all. Begin shifting power up, or speed down, watching for the moment it starts to engrave.
Once you can see anything at all engrave on the device, you either run that exact same setting a few more times (it will get darker each pass), or you bring speed down, or you bring power up. Either of those three options makes the engraving darker, and the computer control ensures you are always running over the exact same path (as long as you don’t move anything).
Practice this gradual increase of power on wood or other scrap material a few times, and you will get a decent sense for how large of changes in speed and power you should be safe making, and what difference to expect from an extra pass.