Back to slack for moment,
I seems to me that the slack at the trunk is mostly for us, for servicing: adjusting, replacing cable/fixtures, fixing squirrel damage. Does anyone mount some slack up and behind the tree close to the fixture to allow diameter growth? I am thinking of a loop at a zip tie or in a junction box?
I agree with James about selection of mature trees as good advice when it is an option, as well as Billy's point to keep after all tree lights with annual service.