Welcome to the Cruising Log Steve
Realistically is 5 years enough time to get sufficient practical experience and training to be able to safely engage in a round to world trip? (assuming that I have to work and can't sail all the time.)
In my opinion you never have enough experience, you always learn because the sea is full of surprises. No course will ever give you the actual cruising experience. Having said that, with some theoretical knowledge and by exercising prudence and common sense you can start your cruising almost at anytime and you will learn on the way. The usual problem with a long cruise such as you are contemplating is not so much the lack of experience but the human factor. A long sailing trip is not everyone's cup of tea. I will strongly advise you to first take a number of small, 1-2 week, cruises with your family to see how everyone likes it, then try a longer one.
Would people view getting to a RYA Yachtmaster certificate as a minimum requirement (or is it more for commercial Skippers)?
RYA offshore is a professional certificate before you get it you must meet some requirements of minimum sea time. It is widely recognized and respected. It is good for the insurance companies. But, almost any course which will give you the basics and some offshore hands-on cruising experience will do.
Wish you the best of luck.