Cutting it into sections would be the best way to sterilize it properly. You could also steam it or create massive ph changes (heavily misting with bleach, cleaning well, then heavily misting with vinegar. Just be sure that they don’t mix, I would buy two new spray bottles at the dollar store just for this.). An organism is extremely unlikely to be able to survive being dried out, then steaming, then a high ph, then a low ph. Any small holes that a bug would be buried in would also likely be sterilized, unless the tree grew around the living bug.
If you’re using native plants you could rinse the roots in bleach and then keep the plant in carbon dioxide for a few days. If you want to use carbon dioxide I outlined one way to do this:
This project sounds super cool, I would love to see it once it’s done and a progression thread would be great.