Expanding on the StephenG's answer.
There is a huge leap between any XXXD and 5D/6D bodies. X denotes any digit used in Canon's designations.
The former are equipped with an APS-C sensor and EF-S mount. The latter are full-frame bodies with EF mount.
Another problem you face when upgrading is that EF lenses are compatible with both EF and EF-S mounts but EF-S lenses can only be used with EF-S bodies.
The reasons are:
- EF-S lenses go deeper into the body because there is more room. Both the sensor and the mirror are smaller.
- There is no need to care about the image quality in corners of the fullframe frame - there is no sensor there, anyway, so the lens's image circle is smaller.
That means, when using an EF-S lens on a full-frame body you would risk the mirror hitting the lens and you would have vignetting (black) in the corners, because the lens's image circle won't cover the full sensor. That's why Canon builds EF-S lenses so they won't mount onto 5D/6D bodies.
On the other hand, using an EF lens on an EF-S body is overkill, you have perfect/good/acceptable signal even in places where you cannot gather the data. If you plan on upgrading to full frame, it is a good way to go, because you can keep the lens. Otherwise you have to buy new lens too. But the lens is likely to serve a different purpose on full frame, because the crop factor will make it look 1.6x wider.
When switching to mirrorless, you have to change all the lenses to EF-M ones or use adapter rings. Otherwise you will push the lens directly to the sensor!
Repeating StephenG's advices:
- Improve your composition skills with your actual gear.
- Improve your lenses and other gear (flash units, tripod, filters,…), if you want to go to full frame, focus on EF lenses. If you are OK with an APS-C sensor, stick to EF-S, the lenses are cheaper and lighter.
- Improve your lenses and other gear.
- Improve your composition skills with actual gear.
- After all that, then upgrade your camera body.
Be sure that you have already found the camera's limits. In other words, be sure that when advancing to next step there is no other way how to improve your images but advancing to the next step.
Do not hesitate to attend workshops dedicated to your favourite style of photography. They will show you what you are doing wrong, where your limits are and how far you can go with your actual gear.