Hi Paul
If you were intending to stick to just private (PPL) flying, then your idea is sound - indeed, you'd probably enjoy the microlights so much that you might not even bother to 'upgrade' to an aeroplane PPL after gaining your microlight licence.
However, as you state that you wish to fly commercially (and by the way 38 isn't too old - yet!), you must bear in mind that microlight hours don't count towards the hours needed for the various hurdles you'll need to jump.
Therefore any small saving in training on microlights initially would end up costing you more in the whole scheme of things.
Also, when talking of becoming a commercial pilot, you must realise that it will be very expensive, so any small savings at PPL level are irrelevant, and often counterproductive. As you no longer have youth on your side, you might well be considering moving through the commercial training faster than others.
So it is worth considering your aim in your flying training. If you are serious about becoming a commercial pilot, then it is probably wise to leave the microlights to one side, and take yourself to a school that offers commercial training, so that your flying training is all focused on your main goal, without any 'wasted' hours that won't count. It will also offer you continuity of training at a school that knows all about training to commercial standards, and one which is regularly exposed to students who want to train straight through to CPL / IR.
If you want a fun, leisurely route to private flying, then go find a microlight school and I doubt you'll want to fly the standard metal 'spam can' after the fun you'll be having in one of today's high performance microlights!
Whichever way, good luck in finding your path to the freedom of the skies...
... from an airline pilot, who started out in gliders and microlights many years ago.