Just to clarify, on your T6i a 50mm lens is a "short telephoto". There wouldn't be a whole of difference with 45mm, 40mm.... Even 35mm is still slightly telephoto.

Sigma makes a 30mm f/1.4 lens that would best serve as a "standard" or "normal" lens on T6i (i.e., not wide angle or telephoto). It ain't cheap, at about $500.

If you want a mildly wide lens, try a 28mm or 24mm. Canon's EF 28mm f/1.8 USM is a nice lens and fairly compact. The EF 28mm f/2.8 IS USM and EF 24mm f/2.8 IS USM lenses have image stabilization, if you want it. All three of those Canon lenses sell for around $500. Or, if you want something super compact and very inexpensive, Canon makes an EF-S 24mm f/2.8 STM "pancake" that's currently selling for $129.

A 20mm or 18mm would be a moderate wide angle. I use Canon EF 20mm f/2.8 USM fairly often. It's quite good, especially on an APS-C camera like the T6i, since this type of camera "crops" the image and only use the best and sharpest center portion of the image.

16mm, 15mm and shorter focal lengths are "very wide" to "ultra wide". There aren't a lot affordable prime lenses in these very short focal lengths.... But there are some good zooms. For example, there is Canon EF-S 10-18mm IS STM for under $300.... Or their somewhat better built and "faster" EF-S 10-22mm USM, which is about twice the price. Both have excellent image quality. There also are Tokina 12-28mm and 11-20mm, Sigma 10-20mm, and Tamron 10-24mm.

Do you have the kit lens that came with the T6i? That's typically is an 18-55mm. You could use that zoom to explore the different possible focal lengths and help you decide what you want.