Hi and welcome to the forum:
Given that the Xt is an interchangeable lens camera, if you are not getting sharp images any more, and you want to keep the camera, it might be worthwhile seeing if you can try another lens attached to the same body.
If it turns out to be the lens, you could easily get a replacement, or much better unit as optics have come a long way in 19 years. What is the precise description of your lens - as you would read it from the front face of the lens?
The Rebel series was Canon's consumer brand, above which where the enthusiast cameras with an ID of ##D e.g. 60D, 70D, 80D; and then the prosumer and professional cameras with the #D designation: 1D, 5D, 7D. Given that the XT model was released in 2005, if it turns out the camera itself is the issue, you will not be able to get it fixed, nor would it be economical to do so. You can get a newer DSLR camera that will be far more powerful, but will feel much like your XT - new or refurbished, but that market is shrinking as Canon move to an other platform.
As far as the EOS M series is concerned, it was at the the consumer and enthusiast levels, but has been completely retired by Canon in favour of the new R-series Mirrorless Interchangeable Lens Cameras (MILCs). They have been around for about 5 years now and this is where the development is, and has an expanding range of bodies and lenses.
If you decide to go for a new camera and lens there is a trade-up program that Canon offers. If you were to choose a new camera, we can help you with advice on what models might be best for your purposes, but let's see how the measure of changing the lens works out first. Replacing the lens will be the cheapest solution. See if you can borrow another lens for even a short time to check if the lens or the body is at fault.
