So, you are growing a trunk. What is your plan? A fat straight tube? A gracefully moving one? With taper or without? Etc. AND, how tall a bonsai are you aiming to make in the end?
Otherwise, of course you can chop it while it is in the ground - just wait until you begin to see buds swell in the spring. Then around the following May/June, select which of the new shoots will be your leader to make the next section of trunk - shorten the others; maybe wire this leader up and on a trajectory that traces how you want the trunk to go if it isn't going where you want. Probably rinse and repeat; but remember the trunk you chopped won't thicken until the next chunk above it is nearly as thick --> don't chop until you've got the thickness/caliper you want.
The last bit is that you'll only get new shoots from a node, so you don't have complete freedom to chop it where ever you please. Also, the final branches can only come from nodes unless you graft branches later on, after your trunk is built --> you will want nodes to get shorter as the trunk develops height.
If you like this tree, you might consider air-layering instead of chop-and-toss.
