You don't need a thin mount filter when using FF capable lenses like yours on a crop sensor camera. In fact, you can probably stack at least two standard filters without any concern. Ditch the silly UV/"protection" filters when doing that... Those serve no real purpose other than making you feel a little more at ease using your pricey lens.
If you want to experiment, there are some less expensive, uncoated variable ND filters available. Just be sure to use them with a lens hood, to help avoid flare issues. The downside to that is you'll not be able to just buy a filter for your largest diameter lens and step it down for use on the smaller diameter lenses. The lens hood will make that difficult.
I'd also suggest starting with a solid ND (or variable, which also provides ND across the entire image).
If you do shoot some situations with bright sky vs foreground, you could use a Graduated ND on top of the ND that's being used to reduce exposure. It's often better to use rectangular Grad ND, anyway, instead of round screw-in. The rectangular ones and their respective holders allow you to slide the filter up or down to position the horizon line where you want it. The screw-in type always position the horizon line across the center of your lens, which, altho they can be rotated a little if needed, essentially dictates how you'll frame your shots.
But, if you only need a stop or so of Grad ND correction, it's often easier to just make two exposures and combine them later in Photoshop.... Or double process a single image and combine them with layers
You don't want the center spot ND... those are actually for use with certain lenses or for special effects. Some very wide lenses are dimmer around the periphery, so a filter is used to even out exposure across the entire frame. You can also use these to induce some deliberate vignetting.