One of the main reasons for using a stool bed is that the "cut all the branches and propagate them" step is exceedingly optimistic.
A more typical timeline is to plant your new rootstock and let it grow for a year to build up reserves, then cut it to the ground. Let it sprout, girdle the sprouts and pile stuff (sawdust, peat, etc.) on top to make stools, then harvest the stools and plant them out and topwork them.
The stool bed keeps making stools as long as you maintain it and don't kill the mother root (or it kills itself, being not the longest-lived rootstock on the planet.)
Propagating rootstocks from hardwood cuttings is exceedingly rare for a reason - it's a VERY high failure approach. If you're in a hurry, buying rootstock in quantity is certainly MUCH faster than propagating up from one rootstock mother plant, and not too expensive from what I see. If not spending money and doing it yourself are more important to you, use several of the initial stools to expand your stool bed, (there's another year with no fruiting scion for those plants) if you anticipate needing lots and lots of rootstocks and don't want to buy them. It won't be fast.
Standard commercial practice, as I understand it, is to discard all the rootstock cuttings - and they would not do that if it were economical to propagate from them. Stool beds are used because they work...
Here's a nice article from 59 years ago on stool beds and layering...
Perhaps if you don't mind the extra labor one of the layering options will work better for you. They do discuss hardwood cuttings, but the process is complex, to say the least, and setting up for it probably will exceed the cost of simply buying enough rootstock.
Here's a bit more recent if you really want to spend a lot of money on the latest equipment/chemicals, instead of rootstock: