For a tail these size, you will likely need at least some stuffing, even if the entire tail is not stuffed. This is so that the shape can be held in place without any under structures showing through the outer fabric.
Here is a tutorial for a different type of tail style, but it doesn’t use stuffing and has a sturdy attachment method. I would also recommend looking through some tail tutorials to gather more ideas. The fursuiting community is also a great resource for tutorials, so if you’ve only looked for cosplay tutorials, I would branch out.
For something like this, you will want to build an armature of sturdy wire in the shape that you want, and use either a layer of batting over it for the plush roundness or a layer of soft upholstery foam for the same effect. The interior can be hollow, which will save on weight. You can then make a fur covering for it and add your attachments.
The duct tape belts that you are planning will unfortunately not be strong enough to hold a bulky and heavy item such as this is place, ad duct tape eventually stretches and wears out. You will want something firm that can hold the tail in place without sagging over time. I would recommend creating a wide belt out of non-stretch fabric to wear under the outfit or using nylon webbing, like backpack straps are made of. Make sure any attachments are padded.
I hope that helps! Good luck :]