It sounds more like a tanekaha. Toatoa would be unlikely to have achieved that scale. Cutting the stem out ruins the normal symmetrical form, and at that height, it's difficult to say whether it would respond favourably to a severe cut. It's understandable about your light, so (if it's a garden plant that was placed inappropriately, rather than on reserve) you could opt for removal (and possibly replace with a smaller native tree) or try chopping it substantially lower and see what happens. It will look unsightly for at least 2 years, after a serious chop.