Design it the same way one would design a portable console.
A wand first and foremost should be modular. There needs to be a solid non-magical base (so no cross contamination) with a lot of shielding for the core swappable components (in this case spells).
Spells should be composed in the same generic fashion with different "models" for different levels of spellcasting. The closer you get to a standardized approach the better it is for everyone. Spells should be pre-packaged with their magi-metals and spell focusing components.
Law enforcement and other users should very quickly be able to determine the level of the spell based on how big/complicated your wand is. Pulling out a level 1 Jacket Cleaning wand will look a lot different than the level 9 Apocalypse wand.
The modular nature allows for swapping of spells, a single moddable core wand, and the specifics of magi-metals can be handled by professionals.
Addendum: Naturally occurring foci.
Given naturally occurring foci exist, not just anyone could proficiently attach it to a wand. The magical people have worked for generations on learning how to tune and control these forces (via magi-metal, vibrations, etc.). A layperson could create a wand with these as a core but, they are highly volatile and not something you would want to do day-to-day without training or good health insurance.
"a wand permits non magical people(or people who have a limited natural ability for spellcasting, aka, general population) to have a reusable way to cast spells, as well as switch between spells being used." The mages would know how to create spells as well as collect the proper natural foci. How else would they cheaply make wands! The especially potent ones they would naturally keep.
Additionally, if you can't properly feel magic you can't identify natural foci compared to normal quartz rocks. You can't make normal quartz rocks attune to magic without being able to use it either. So natural foci exist and are used but, they are dangerous unless prepared by a mage.