"It is the "moving" which causes the change in depth of field. Depth of field is determined primarily by three things.
#1 Focal length of the lens (not the equivalent focal length... the true focal length)
#2 Focal ratio of the lens
#3 Subject distance from the camera (specifically from the camera's image plane)."
Perhaps an AOV chart will help you understand. Here is one for you to check out.
I feel the 80D with the ef 24-70mm f2.8L is the best choice considering price and quality. That is why I suggested it. I still think so in spite of your rather protracted reply. I could go for the ef-s 17-55mm f/2.8 IS USM Lens if the cost is still too much.
I also imagine the OP will use the gear for more than just portraits. That is why the primes have no place here. Not for starters.
"...either the EF-S 18-55mm f/3.5-5.6 STM or the EF-S 18-135mm f/3.5-5.6 STM..." No not never in this lifetime would I even mention either of these let alone consider one.
Now, I suppose the OP is totally confused and probably abandoned this thread for no good reason as what you try to lay on folks simply doesn't exist. Do you know how many weddings, Senior photos and portraits I have done with a 1D, 1D Mk II, 1D Mk III and 1D Mk IV, yes, crop sensor cameras? I made a living doing that. If you pick the right lens with the right AOV (look at the chart, my friend) all will be right with the world.
There is no doubt you are a great mind in astrophotography. You make beautiful pictures but you are over thinking this part of photography. The problem simply doesn't exist.
Tim said,
This is NOT to say that you can't take beatiful portraits with a crop sensor camera. You can... but you have to know what the crop-sensor is going to do to the depth of field and what you can do to compensate."
This is where I believe we conflict. The AOV will be the same however you will need to open the aperture to achieve the same DOF. Let's say you were shooting at f8. You would need to use f5 but the AOV is the same. This is because a lens is a lens, is a lens and can not change its physical characteristics. Say a 50mm vs a 30mm for instance. But nothing is cropped.
Of course the photographer could just, heaven forbid, move a bit and not need to change aperture.
And this supposes there is some inherent need to compare your Rebel camera to a FF camera. Consider the photographer that never uses or has used a FF camera. Do they need to compare or should they just learn how to use their Rebel?
You are comparing apples to oranges, Ernie. You are comparing different focal lengths and aperture settings. Your “facts” are at best very misleading.
I think a better example would be using the same aperture on the different camera bodies. Look at the results for your 10 foot distance at f/8 now.
First the APS-C body. DOF is almost four feet.
Next, the full frame body. DOF is about 1.5 feet.
Significant difference. The portrait photographer would want to have the smaller DOF.
Work? I changed the spelling a bit to suit my needs, I now spell it like this, p-l-a-y. 
"Does this mean we agree?" It does if you admit the same photo can be made with either format camera if the right lens is selected. That is my main most point. Whether you adjust the f-ratio or you move your feet. The exact same DOF can be possible, actually no matter which camera format you use. Physical limits of manufacture, of course apply.
Consider a person, new to photography, doesn't own or have any desire to own a FF DSLR. They have no interest or need to know what 1.6x is. They simply need to know how their camera operates and functions with their lens(s).
IMHO, crop factor is a term that should have never been created. Especially since 'crop sensor' cameras crop nothing.
