Can I use a tuner like the LDG AT-100 Pro with the type of vertical you built, or do I have to go with an Icom AH-4 mounted at the antenna?
No reason why you have to use the Icom tuner, but for best results if you’re going to go with LDG Electronics you should use their remote mounting RT-11, not an indoor tuner. You’ll also need one of LDG’s baluns in-between the RT-11 and the antenna. Even better, use one of the SGC autotuners like the SG-237, they are ideal for this application.