This assumes one would own an acquired slave forever, which is an assumption not regularly made for, say, a tractor. Plenty of flexibility could be achieved through a vibrant slave market. You sell slaves when you don’t need them and buy more as needed. Freely formed slave prices would quickly adjust to the market’s supply/demand of/for them, just like for anything else in the market.
Here’s a thread on the idea of personal incorporation (i.e. ownership/market in shares of others) which would cover the grey zone between 100% self-ownership and outright slavery (0% self-ownership). Decent brain-gymnastics, I thought:
https://forum.freecapitalists.org/t/the-unincorporated-man-book/9826
Z.