There are a few vital components missing from your question. It's hard to offer meaningful advice without certain criteria.

Firstly, what do you feel is lacking in your present equipment and why are you not satisfied with the results you are producing with it? If you are not pleased with your results, do you feel it is due to limitations in your present gear or can it be related to your technique? What is your output- do you routinely make large prints that require maximum sharpness and resolution or are you viewing your work on a small or moderately large screen?

Too many photographers are bitten by the gear bug and feel the answer to their passion for photography lies in constantly upgrading their equipment. When I hear statements like mirrorless cameras being the "wave of the future" I become suspicious! Your 'future" in becoming a better photographer is dependent on your artistic talent, you work ethic and increasing your knowledge base in technique, composition and seeing light.

Granted, you need good tools to do the work and if you really feel your equipment is inadequate and it is limiting your potential, is too cumbersome or heavy for the tasks you require, and it is not appropriate for the output you are producing, by all means, make the necessary investment.

If you are specializing in landscape photography, unless you are mountain climbing or operating in extremely difficult terrain, you can take your time to compose, use a sturdy and stable tripod, adjust your camera and expose carefully. If you are not shooting fleeting wildlife or rapidly moving action such as in sports photography, camera weight and speed of operation is not a major concern. If you are aspiring to fine detail and large image display, a full-frame or even a medium format digital camera may be an important asset and well worth the investment.

For traditional landscape photography, a focal length just slightly longer than normal for your format is advisable for "as the eye sees it" perspective. Of course, if you are into othere alternatives as to distance and perspective, wide-angle views, panoramic compositions or more compressed perspectives, wide-angle and long focal lengths should be added to you compliment of lenses. 600mm is not your standard focal length for landscape work but should serve you well for wildlife coverage and special effects.

DSLR vs. Mirrorless? After 55+ years as a professional photograher, I can honestly tell you a camer is a camera! In any given price point and category, all things being equal, there will be no difference in image quality between each of theses configurations in cameras of the same level of quality with good glass upfront. Your choices will be a matter of ergonomics, camera handling, weight, portability viewfinder access and type (optical or electronic) and budget.

Personally, I opt for gear that is not overly engineered with too many bells and whistles. I don't like stuff that becomes cumbersome and gets between me and my subjects. If you take the time to read many of the questions, here on the forum, you will notice there are many requests for advice on troubleshooting issues and problems with gear performance. Many of these problems stem from too much technology- too many menu settings that tend to go wrong. When you get right down to brass tacks a camera has 4 settings; aperture, shutter speed, focus, and ISO sensitivity. If you get those down pat, understand all the relationships you are good to go. So many photographers are too preoccupied with their gear, fumble with the equipment and thereby cannot concentrate on the artistry. A decent body with a good metering system, a few convenient programs such as aperture and shutter priority, manual mode for finite control and a fully automatic program for just fun or impromptu shooting will do the trick.

I was at my dealer's shop the other day. He showed me the latest and greatest camera. The menu looked like the instrumentation on the flight deck of a 737! Not for me! Think about it!