A few things to think about....
Wide angle is not often used for portraits, in particular. The inherent perspective distortion effects of a wide lens can cause problems in shots of people. You have to get in closer with a wide lens, and when you do that a person's nose will look unnaturally large while their ears look oddly small. This isn't a hard and fast rule (not much in photography is)... Wide lenses can be use for humorous portraits or for wider shots of the person in their surroundings, "environmental portraits", such as showing them in a favorite place or at work or, perhaps, a small group of people doing something. You also have to be careful positioning people near the edge of the image with a wide lens... That can cause anamorphic distortion or "stretching", for example making one of their hands or arms look much larger than the other.
You have an 18-55mm lens, so can experiment with wide angle effects in telephotos with that.
So, in general, normal to short telephotos are more commonly used for portraiture. On your crop camera, 50mm to 85mm is the range of "traditional" portrait focal lengths. Your 50/1.8 is a good intro lens for portraiture and your 70-300 can provide an opportunity to try slightly longer focal lengths for the purpose.
Longer telephotos also impose some perspective effects... though their compression effect is considerably more subtle than the effects of a wide lens. Longer focal lengths are deliberately used for some types of portraiture... fashion photography, for example. A longer focal length does require much more working room and/or make for a tighter framing. On your crop camera, 100mm, 135mm and up would be considered longer portrait teles.
Another desirable feature for a portrait lens is a large aperture.... Mostly to be able to blur down backgrounds. This isn't always necessary, if for example you are only doing studio portraits and have a lot of control over the background. But shooting candids on location, where you have less control over what's behind the subject, it can be very handy to have a large aperture lens. Your 50/1.8 is large aperture, but your 18-55, 70-300 and the 24-105 are not. But, again, depending upon how and where you shoot portraits, this may or may not matter. Big apertures also generally mean more expensive lenses, and sometimes larger and heavier as well.
I understand you thinking that you would prefer not to buy lenses twice, plan to get a FF camera sometime in the future, so would prefer to only buy lenses that are compatible with both. Been there, done that. It's pretty easy to do, with mildly wide to standard to telephoto lenses. However, when it comes to wide you will pay a premium and get less unless you are open to a "crop only" lens (probably a zoom, since there are very few primes that are particularly wide on a crop camera). If/when you ever do go to a full frame camera and can no longer use a quality wide angle lens, you can sell it and in some cases recoup much of your original purchase price (e.g., I bought a Tokina 12-24 around four years ago and today they are selling used for only a little less than I paid for it new... other lenses I bought used years ago, I could sell at a profit today). There are no guarantees.... but it often works out pretty favorably. Quality lenses, in particular, generally do not depreciate very much or very rapidly.
One other thing... Since you are trying to plan around it, you might give serious consideration whether or not you'll ever really need to go to a FF camera. Yes, there are some advantages to FF... But there are also advantages to crop cameras, which have improved in leaps and bounds, perhaps more rapidly than FF cameras. We are in the 4th or 5th generation of Canon FF cameras now... But in the same time frame have probably seen 8 or 10 or more generations of crop cameras. All DSLRs have steadily improved, but crop cameras in particular have come a long way and will likely just keep getting better and better.
I use both crop and FF, for different purposes. Unless you make huge prints, or need really low light capabilities, or can cite other significant reason for full frame... current crop cameras come surprisingly close to matching FF in many respects. Crop cameras offer more lens choices because they can use all Canon EF and EF-S lenses. FF are limited to FF-capable EF lenses only. Lenses for crop can be a lot smaller, lighter and less expensive. Compare a 300/4 IS on a crop camera with a 500/4 IS on a FF camera, for example. I don't know about you, but I'm not getting any younger and smaller, lighter gear is appealing more and more to me each year.
Okay.... all that out of the way... you were looking for lens recommendations....
Though sometimes I use other lenses and these serve other purposes too, for portraiture my most frequently used lenses are:
Primes for crop: Canon 20/2.8, 28/1.8, 50/1.4 and 85/1.8
Primes for FF: Canon 28/1.8, 50/1.4, 85/1.8, 135/2
Zoom for crop: Canon 24-70/2.8
Zoom for FF: 70-200/2.8 IS
But, again, there isn't some hard and fast rule... I use a variety of other lenses for "portraiture". Depends upon the situation. For example, I shoot a lot of equestrian events and always try to get both action shots and closer/tighter portraits of the participants. For those most often I use 70-200 on one crop camera and a 300mm on another. There are times at those events when I use 12-24, 24-70, 28-135 and/or some of the shorter primes listed above, too. Other exceptions: For wildlife "portraits", I use 300mm and even longer. For insect "portraits", I use 100 and 180mm macro lenses.
Sorry, I don't have and haven't used the Sigma 150-500 or Tamron 200-500... I have heard pretty good reports on the Sigma 150-500 OS and it's companion 120-400 OS. They seem to compare pretty well with the Canon 100-400 IS.
You might want to check out the Lens Photo Sample Archive sub-forum attached to this one. You can search for any particular lens model and see what people are getting with it, as well as some user comments.
