I am also interested in how to encourage thicker trunks but I can share my propagation experience as I have already done that.
Here is what we have done to our Draceana marginata which is much smaller than yours. It had three branches which we have cut off each. So we did not cut the main trunk but the branches itself. These branches were quite long so we have further cut them in half.
This gave us 6 cuttings out of which were 3 tip cuttings with the leaves and 3 cuttings without leaves.
We have started to root the tip cutting in LECA (Lightweight expanded clay aggregate) aka clay balls. It is better for rooting than perlite as it retains moisture better. (For more: What is Leca and why is it useful for plants?) We have put the leafless sticks into water.
All of the tip cuttings in LECA rooted really well, while we had succeed rooting only one stick in water.
The trunk of the mother plant started to push out new leaves pretty soon. One stock on each branch except one where there is a second one coming in now with some delay.
Note that we have did it in the middle of the growing season when we could supply the plant and the cuttings with plenty of bright indirect light. So I would not cut the plant until summer.
Here are some recent pictures with the cuttings planted back next to the mother plant.
||the full plant