This is likely to be one of the L. spectabile hybrids, possibly Leptospermum 'mesmereyes' which is a bit more upright.
I know a couple of members have posted about probems cutting roots of Leptospermum but I have not experienced any so far

with the ones I am using. I have pruned off up to 3/4 of the roots without any problems (mostly Nov, Dec and Jan). If you have not done so yet, start by scraping the surface until you find where the main surface roots grow from the trunk. Cutting blindly through the rootball can be a disaster if the plant was potted deep last time it was potted on.
Some of the Leptos will bud back on bare wood while others do not. If in doubt I would cut back to the lowest live shoot. That will usually be enough to stimulate budding further down if the variety is able to. Then you can cut further when the buds have grown a bit. If it does not bud you will still have a live tree, even if it is abit bigger than planned.