The Right Honourable Jimmy

It should be fine, the rx 580 is sufficient however I would suggest a stronger cpu with more cores (either coffee lake or ryzen), however I do understand that it is a mac, and as long as you don't try anything too complex it should be fine

dude this is really not how it works! Each of the cameras above could be "the best" for a particular usage, for different reasons. 
 
I see no point in the t6i being compared to the t7i, since the latter will obviously have advantages over the former (in this particular instance, the t7i is not an incrimental upgrade over the t6i either...).
 
The nikons are an incremental upgrade of each other, the d5600 being the newer and best of the 3. 
 
The a7 (I assume you are talking about the original) has a FF sensor for God's sake and is an entire different market segment aimed at entirely different people. It comes with many advantages and disadvantages over the others. 
 
The same sort of goes for the Olympus, although not entirely. 
 
So your choice is really between the A7, t7i, Olympus and d5600. And at this point in time you could go for anyone of them for a different set of reasons.
 
The only case I will make is between the d5600 vs t7i, where I would recommend the t7i, just because at this point in time it is newer and has several significant advantages over the nikon like DPAF, a far better stills AF system and newer processor. (I would probably not say the same when the d5700 or whatever it will be called is released...)   
 
Other than that, the a7 and Olympus would be "the best" for any set of reasons. It is not so clear cut. 

The High Sierra beta supports any graphics card with current mac drivers. The nvidia 10 series cards do work, but Apple is working with AMD currently so that's what they advertised in the WWDC keynote. I know some people who have tested a few different cards. That being said, I would probably wait until High Sierra is released and see what eGPU enclosure is best for the price.
 
 I use exFAT for all my externals because I frequently go between mac and windows, and use them for video editing. Every so often I hear someone say "exFAT is bad!" but I can't ever find any real proof. Anecdotally, it's been working fine for me for like.. 8 years or so? One of my hobbies is editing video, and another is testing encoding/transcoding settings between systems so i regularly write decent sized files to them.