Time to resurrect this moldy old corpse and infuse it with some dark necromantic energy. Adding markers is very easy, the M key. You notice markers are numbered. Sometimes you might want to give them a label, other times the marker itself is all you need. What inserting markers DOESN'T do is reorder the numbers if you insert them non-sequentially. You can see an example here. It just looks... weird. Good thing though, is there is an action to renumber all markers. Two actions, actually, doesn't matter which one you use. Let's take a look.
So from here it's simplicity itself to make a custom insert marker action that first inserts the marker, then renumbers all the markers. That way your makers will always be in order.