AusBonsai

I have seen some extreme root pruning for trident maples and elms. Does anyone know if the same can be done to Myrtles?
There is a beautiful myrtle growing in the garden for the last 5 years and I think will make a great pre-bonsai. The problem is, when I gave it a push, it seems to be solidly anchored.
It is a dwarf specie, around 1m high with a trunk of around 7-9cm.

The buds have not started moving. I quick dig around the plant didn't expose much surface roots so I am thinking they must be straight down.
What would be the best approach to transforming this tree into a pre-bonsai?

I am thinking to wait for the buds to swell, then dig around 15cm on one side of the tree and cut off any large roots. Also give the top a prune to match.
Then the following season, depending on how it responses, dig the other side of the tree and do the same.

Does this sound like a safe option or can I just do the whole tree in one season?

I am new to bonsai and new to this site. Thanks in advance for all the great articles that I've read. Hopefully, I can contribute to this resource in the near future.

Crepe myrtle is remarkable tough and can handle root reduction well. I would do such a tree in one go - dig, prune roots as hard as necessary and cut the top back then pot up all in 1 go.

Thanks Shibui.
We have met in August when I came to pay you a visit which was truly enlightening. A short chat with you is like a life time of knowledge.
I know you hard root prune your tridents in mid/late winter. Do you think it is too late to do it to the Myrtle now? I have not seen any buds movement as mentioned, but this does not mean activities have not started below.

No buds means the tree has time to heal and produce more roots before it needs water for new leaves so no buds means OK to prune roots Dennis. I would do it now.

My crepe are on starting to Bud this week here, so if I had not already repotted, they could be done now.
Root prune as hard as you like.

Ken