Given your experience, I recommend you read the following document and learn.
Equivalence: considering the impact of sensor size
You have moved from an APS-C camera to a full-frame camera and that means that any lenses (EF-S) that you attach to your camera will not project an image onto the full area of the sensor. This has a significant impact on the MP captured: because the crop factor is 1.6, and because we are talking area, that reduction is the square of the crop factor: reducing the MP potential from 24MP to just under 9.3MP. The camera senses that an APS-C lens is being used and reduces the recorded area automatically to prevent a large vignette. It is clearly stated as such in the Advanced User Guide on P1051- which is your best friend: PLEASE USE IT!
The whole point of JPG is to take the large RAW image that the camera actually records at the sensor and processes it in camera to apply some image adjustments before saving it, but that process throws away a significant proportion of the image size to produce a smaller image. That might be acceptable under some circumstances, but if you need to process your images out of camera to make large, high-quality prints, then you really want that RAW file format and your PP software will have much, much more to play with.
RAW images are meant to be processed: and the result of that could be a very large file in the form of a TIFF or PNG file that can be used for printing. The quality of the resultant image will depend at least as much on the lenses you use - certainly, they are the longer-term investment.
Given you say you create large prints: what size prints were you considering creating and for what purpose - for competition, to mount at home, or for sale???
I recognize that you are frustrated, but there are two sets of things you need to master:
1. The principles of photography - how light is metered and how to create well-exposed images using the 'holy trinity' of controls: ISO, shutter and aperture. While many combinations of these will produce a correctly exposed image, each will produce different artistic and compositional results. Those truths are universal to photography.
2. You need to spend the time to get to know the controls of your camera. Modern MILCs these days have those three controls buried in a host of other, high-end features, such as different focusing modes, tracking, exposure configurations etc. In starting out, go back to basics and spend time with your manual - yes, the manual is there for a purpose, and watch videos like the one I offered you before.
Don't spend your time, money and patience printing until you know you are getting the results digitally that you want, and then try a few tests. You have changed brands and also camera types and that is a learning curve you can't short circuit. You need to have patience, but it will be richly rewarded as you learn more about photography and also your camera.
