I think it depends on the root system it has and the trunk size you want. If you want a thick trunk you should do that first (put in ground/huge pot). When the trunk is good you should train the roots so it has enough feeder roots and fits the bonsaipot (remove taproot/big roots). Sometimes when u have good material it's possible to do it in 1 go. (but you need trunk-fatness first and enough roots, or a strong tree/species).
A lot of stuff that is neglected nursery material that i get has had a lot of soil washed out of the tops of pots and the roots have made a thinner and tighter compact root ball near the trunk. When I get em out in most cases there are large sections of dead roots corresponding to the die back on top and some dry sections where the roots aren't getting anything anyway. I can usually trim back to healthier roots and then with really good after care the trees usually respond well since before they were getting nothing - no fertilizer barely any water etc. I also live in a pretty warm climate so I can get the trees nice and humid and wet to promote growth/recovery. Plus I prefer to do the most aggressive work to the roots only once so that it can spend more time in a pot without needing a repot for a while. While I wouldn't recommend it for everyone I have taken 15 to 30 gallon trees into pretty small boxes and shallow pots in one shot. I also get material pretty cheap and have access to a lot of it.
