The maps I supply are datum/position format agnostic (means that the maps are not set to a specific datum or position format).

You can use whatever map datum you like, but bear in mind that many (mainly Grid Based systems) are only suitable for specific geographical areas, such as British Grid (this is no use elsewhere and can actually be dangerous to use outside the UK as the accuracy will be increasingly out as you move further away from the British Isles)

All grid systems are only designed to be accurate in the country they were created for. If you try and use them outside the intended area of use, the accuracy drops off very quickly, making them at the very least less useful, and at the worst highly inaccurate and dangerous.

Personally, I use WGS84 as my preferred datum as this is accurate anywhere in the world as it was designed for use across the world, not just a small geographic region. As to position format, I personally use: hddd° mm.mmmm’ WGS84 is as accurate in New Orleans as it is in Copenhagen, Paris, Madrid, London, Abu Dhabi, Dohar, etc.

There are a full list of the Datums supported on most Garmin devices, here: https://support.garmin.com/en-GB/?faq=CuRXNp4JBU25QHE8cwRI26

As to position format for use with WGS84, you should use either UTM UPS, MGRS, hddd.dddddd, hddd° mm.mmmm’, or hddd° mm’ss,ss” (all of those are as accurate anywhere in the world) or if you prefer, the correct Grid system for the country/region that you are going to use the GPS device in.