Nagami Cumquat Tree

Laney starts with ...
Need some help please. Have 2 Nagami cumquat trees approx. 15 years old in the ground which are around 2.5 metres tall now. There is healthy foliage on the top 1 metre of the trees but the bottom 1.5 metres are just branches with no leaves at all, possibly to do with the trees not getting total full sun plus I do prune off the dead wood each year- they would get around 60-70% sunlight. They are beginning to look a little unsightly with no foliage at the bottom and was wondering if I cut them back to about 1 metre in height, if they would shoot again. Really do not want to pull them out - I am just a little nervous about pruning them back so hard. Would like your advice PLEASE!! Thank you so much Laney