You're welcome, Mark! I looked at the weather forecast for San Diego and it seems to be okay for cutting roots at this time.
To get the fat caudex/root extensions, I cut the tap root when my seedlings were between 6 to 8 weeks old and had 5 to 6 sets of true leaves. But then, my weather makes the seedlings grow faster so perhaps for your climate or situation, older than 8 weeks would probably be okay.
Here's a couple of trees that I had cut the tap roots around 4 to 5 months old.
The following is an example of root growth after cutting off
all the roots. I think it was 5 months old at the time I sliced off at the widest part of the caudex.
If I had wanted to I could have trained the roots to look like this one (not mine)...too much work.