Yeah the 5/8 wave already has a loading coil in the base and it is hard to predict what the voltages across it may be when operated on other bands with external loading coils. If it were me I would put up a single vertical as long as I could get it, lay some radials on the ground or just under the sod, and use a single loading coil at the base with taps for each band.
Is the loading coil in that base determine the power it can handle or what and if so Channing it to a 10ga or 8ga bring up that capability as well as the thickness make it better in being a bit more broadbanded? Speaking single band and multiband?
I am not the smartest cookie in the jar but I do have wondering thoughts on things and nothing like asking if you don't you'll never learn ever.