I spend alot of time driving back roads for work I just got a rebel 7 and I'm trying to find a lense for taking picture of bears and other animals from about 600 meters and have a fairly close up pictures of the animal. Ideally I'd like to use the Camera in place if binoculars.
Without spending orders of magnitudes more on the lens than on the body I know of two options. Both having the disadvantage of requiring manual focus on your body.
The first option that I have tried is to buy a mirror lens, for example the 1000mm MTO-11ca that I have. This can be handheld in good light at least on my canon 5d mark II. You will need shutterspeeds of about 1/2000 or faster to avoid camera shake at f/10. So sunny day works, cloudy day needs tripod. (a variant on this is to buy a spotting scope or a small telescope and mount the camera to that)
The second option is to buy an 150-600mm zoom lens (sigma and tamron have options). On its own this will make the bear quite small in the frame but if you are ok with loosing autofocus you can combine that with a 1.4x or 2x teleconverter for good image quality according to the tests I have seen. I have not shot with this myself but the resulting aperture is similar to the mirror lens. If you are willing to upgrade the body as well some newer canon cameras can autofocus at such small fnumbers (that eos r should do it, maybe the m6 mark ii you would have to check).
tl;dr: You can't do that on your budget.
An 800mm lens on a Canon APS-C camera like the Rebel 7 has a field of view of 1.6°. At 600m, that corresponds to 2 * tan(1.6°/2) * 600m = about 20m, or in other words the ~2m long bear will take up about 1/10th of the width of the frame. You might be able to crop the image down at that level and get something useable, but only just. You're not going to be able to make do with anything shorter than 800mm.
For what it's worth, the Canon EF 800mm lens costs significantly more than $10,000, weighs 4.5kg and is just under 0.5m long; Sigma have one for maybe half that price.
(As a technical note, you probably don't want a zoom lens - one which can change focal length - you just want a "telephoto lens").