My 160 m antenna is 130 ft long. It goes up 50 ft. then slopes down to 30 ft. on the far end.
I can add another 300 ft. making a cantilevered antenna a bit over 400 ft long probably 20 ft or so high at the mid point.
Top loading is certainly useful, but currents in a wire sloping downwards will be
out of phase with the current in the vertical section. I read an analysis years ago
on a scholarly journal reporting that the top hat shouldn't extend down more than
half the height of the antenna, regardless of the length. (Actually their number
was 55%, but I don't remember if that was up from the base or down from the
top.)
So however much wire you can install with the far end above ~25' may be
close to optimum.