"be hugely costly and still only result in a license that isn’t immediately transferable to other countries (eg you have to hold a license for x years before you can drive abroad and/or older than x years old)"

So I'm going to ask again, what kind of countries issue you with a licence that isn't immediately transferable to other countries? You can get an international driving permit literally the same day you pick up your regular licence, there is no time limitation on that - or if there is where you live, please educate me so I know better.

But a couple of years is a huge amount of time. If you put in just 4 hours a week you'll be done in no time. Not knowing how to drive in that situation is because of not caring very much, not because of moving around. And the vast majority of what you learn stays relevant when you move.