Very good. That is the graft union. You have plenty of trunk (two) to grow a new tree.
Get some good hand pruners and cut the root stock sprouts at the base flush to the trunk. We want sharp clean tools. Make a clean cut on the trunk with no top. I would be inclined to cut the branching trunk just below the bottom of the window, below the split. I'd cut off the lower branch leaving as much of a branch stub as possible. Keep the thorny root stock spots from growing and leave the tree top alone. Come back next year, let's go for Halloween 2022 and we can discuss selective reduction of the Legendary Limes new growth and bone structure.
Harvest your limes. Find a recipe for freezing the lime juice. Save some for Halloween 2022. In just three or four years we can make you a lime.
Q