"...I do want to use it in low light conditions. Gymnastics, dance recitals etc..." Amadeus there are precious few genres where equipment trumps even skill and expertise... Sadly indoor sports; gymnastics, dance etc is one of those very rare scenarios. The barrier to entry in professional caliber indoor sports photography often can be very deep pockets. If you want to shoot commercially in that genre it may take years to recover your initial investment, if at all.

I'm only the messenger here Amadeus; that said I shoot League Soccer commercially and when clients ask if I would accept indoor sports assignments I politely decline... virtually a bottomless pit of low paying gigs and gaining credentials to shoot at major college events (let alone major league events) is nearly impossible (the line goes around the block).

My best suggestion for you is to invest quality time in the UHH Sport's Photography Forum.
https://www.uglyhedgehog.com/s-103-1.html

Now that I better understand your initial query on the Sigma 18-35 f1.8 optic I realize your actual requirement i.e. "need" is greater then a single piece of kit. Please do yourself a favor and hang around in the UHH Short's Photography Forum for several months and then possibly reevaluate your situation. While there are many genres... you kind sir are looking to pursue one of the most complex and demanding of all. Not only do you have to have a high degree of knowledge and savvy in Dance and Gymnastics but also the highly sophisticated kit to achieve excellence.

Keep in mind that photographic excellence is not a consumer commodity but rather a result of long and arduous dedication to a craft.

Please realize that I'm not trying to discourage your choice of genre but rather attempting to steer you in the right direction.

Wishing you all the best on your photographic journey Amadeus

You're certainly causing a bit of confusion in this response.

The Canon EF 17-40 f/4L is a full frame lens. Not as fast aperture as the Sigma option, but rather similar in effective focal length.

The "Sigma 18-35mm f/1.8 DC HSM Art" being discussed is yes, an APS-C lens, that is one that throws a smaller image circle that covers just the cropped-size sensor. The third-party lenses do not use Canon's EF-S mount. Rather, this Sigma lens mounts as an EF lens, the red-dot alignment mark on the camera body. The 'DC' designation from Sigma indicates an APS-C style lens, it will vignette of mounted to a full-frame camera.

For your lens choices, note the 18-35 is roughly just 24mm to 50mm for a full-frame zoom. This is a nice view for sports / action, if you can get close very close to the action, such as under the basket in basketball. You can look at your current results from your 18-135, how often are you shooting at the wider 18-35mm end of this zoom? The Sigma lens is unique in the max aperture for this zoom range, but is that wide focal length really what you need? For the sports you listed, I would think as a spectator photographer, the 50 and 85 are the better choices. The wider focal lengths can give a perspective from 'being in the action', like there under the basket or on the bench or in the huddle. But, the photographer needs to be 'in' these positions to make use of these focal lengths and to capture these perspectives.

Rather than looking for a new lens to replace an already excellent Canon lens, we should look at the actual images and determine what happened in this images. The f/1.4 aperture is a rather narrow depth of field. Where the AF fell, the details should be rather sharp, quickly falling away into a blur outside that depth of field. This applies for a close subject, where at a distance, the DOF increases.

Example, your 50mm on an EOS 80D for a subject distance of 8-feet is just 0.4-feet (let's call it 5-inches). If we increase the subject distance to 20-feet, the DOF increases to 2.6-feet (let's call it 30-inches). We'd also have to look at the shutterspeed, the AF location and the AF mode, to see if we could detect issues causing "everything was soft, nothing sharp".