Frankly, you don't need a new lens, although there's certainly better glass to be had than what's in your bag.
You need lighting gear.
Portraits are basically made by the lighting, not so much the camera and lens (see the Strobist and Tangents). And the idea that better gear gets you a better keeper rate doesn't always hold true. Better sharpness, lower chromatic aberration, faster autofocus, higher frame rate--these things might help you get a better keeper rate, or at least cut down on your post-processing time. But generally it's more about things like timing and composition, and learning to edit in your head before you snap the shot and that's not up to the gear so much as it's up to what's four inches behind the lens.
You may also need more experience or some training. Consider spending the cash on books, videos, a class, or seminar. Or possibly support gear, or post-processing software. Or a better backup scheme. Or backup gear (if you're really going pro).
I'd also say at present, you may have too many lenses. And that you're not prepared for the move to full frame. Here's why:
- 10-18mm f4.5-5.6 IS STM
- 24mm f/2.8 STM
- 18-55mm f/3.5-5.6 IS STM
You forgot a vital portion of these lens names: EF-S. These are EF-S lenses, designed for crop sensor use. The image circle they project is big enough to cover the sensor of a dRebel, but won't be big enough to cover the sensor of a 6D--for that you need EF lenses. EF-S lenses won't physically mount on a 6D. And the only EF-S lens you own where you're covered in full frame terms is the 18-55 (with your EF 28-135. Which, alas, will show more of its flaws when you can actually see the edge performance).
You have no EF 17-40L to replace the EF-S 10-18. Or an EF 35/2 IS USM or EF 40/2.8 STM to replace the EF-S 24/2.8 STM. Remember, when you move to full frame, you have to do the crop math in the other direction--all your lenses get 1.6x shorter/wider. So your 50/1.8 is gonna look more like 28mm currently does on your T5i. So, you might also want to swap it for an EF 85/1.8 USM.
If you really are contemplating the move to full frame, rather than a higher-end crop body, you must also factor in the expense of swapping out at least half your lenses, and any new lens you plan to purchase should probably be EF, not EF-S.
But in addition to all that, you have to be concrete in what it is you want from a lens--what specific features you're after your current glass doesn't give you--where your current lenses really frustrate you, before anyone can narrow down their recommendations to something meaningful.
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Addendum, for new edit.
L lenses may deliver what you want as a pro--especially if you can write them off on the taxes as a business expense, but most of the expense is for larger max. apertures, build quality, and exotic elements (flourite or extra-low dispersion glass). L lenses may or may not be sharper than their non-L counterparts--it depends on the individual lens. Ls do tend to have more contrast so can look sharper/more saturated. Kinda like Zeiss lenses (which you do not want, because they're all manual focus). :) However, for most hobbyists, an L is a very hard sell because the image quality gains can be marginal and the expense is very high. Buyer's remorse can hit harder with an L because it's still just a lens, not magic. OTOH, for a pro, where time is money not having to do as much post correction can make an L easily worth the expense.
For alternatives, again, it depends on the individual lens. But most folks will stick with OEM because of the CPS and generally higher quality. There are exceptions, but it's individual. Most pros with full frame simply go with a 24-70/2.8 and 70-200/2.8 setup, and work their way outwards from there.
If, however, you want to ease your way up via a crop body, then consider a EF-S 17-55/2.8 (crop version of the 24-70/2.8), and maybe a good mid-grade fast portrait prime, like the EF 85/1.8 USM or EF 100/2 USM which will still work on full-frame. Both autofocus very quickly, unlike a macro lens that has to search through the macro range as well as normal subject range to lock. If you want to go for overkill and your first L, you could also consider the EF 100mm f/2.8L IS USM Macro, which has a focus limit switch on it.
