So I could pot up when required until I reach the desired size, then pot down gradually into training pots until the root system is small enough to fit in a display pot hopefully, correct?
The nursery industry calls it 'slip potting'. When a plant has grown to the limit of the smaller pot it is simply slipped out of the small pot and the entire root ball is then potted intact into a larger pot. Provided this is done before the roots start to circle and tangle they just keep growing out into new soil and the plant keeps growing. Plants can be slip potted any time of year as we are not interfering with roots. Even if some of the old soil falls off during the transfer there's very little risk.
Not sure what willow you have but the ones I know grow very fast and may need up potting several times each year. You could probably start with a 6" pot and then go to a 10" or maybe straight to the 14" if they are doing real well by then.
At the other end it is not necessary to gradually downsize. Most trees will cope with a relatively large root reduction. I regularly reduce roots by up to 80% and sometimes more to get trees back down to bonsai sized root systems. All the willows I know will laugh at massive root reduction and just keep growing.
You may find you need to reduce the size of the pot before the final stages of branch and structure development to limit internode length so you can build good, dense ramification. Intermediate sized pots can also allow extra growth which will help healing if you've had to make some larger chops after the fast grow phase.
Unfortunately there's never just one clear road to follow when developing bonsai. Different species grow differently. Trees behave slightly different de[pending where and how they are grown and often a range of different approaches will bring us to a similar finishing point.