Disclaimer: I've been an Olympus user for a bit over a decade, and bought a used GFX 50S a few months ago to become a dual-system user for the first time. While I've done a paid job or two in the past, I am not a professional photographer.
Being on the two "extreme" ends of the sensor spectrum has been pretty eye-opening to me, and it makes me think that there are a lot of misconceptions that are bandied about the photography enthusiast realm. A big one, for example, is the idea that larger sensor = shallow depth of field. My GFX 50S with the 63mm f/2.8 gives the same depth of field wide-open as my Olympus with the 25mm f/1.2, but the Olympus at f/16 (minimum aperture) gives a depth of field that is greater than the 63mm at f/32 (minimum aperture) by about three feet. Depth of field control is thus greater with a smaller sensor... but if Fuji made a 63mm lens with an aperture wider than f/2.8 then you could argue that the shallowness of the depth of field was certainly more shallow with the Fuji. I'd argue that there is finer control over depth of field with a larger sensor (less depth added per f-stop). The semantics matter because it lays bare the other misconception that seems to plague photography, which is that the more shallow the DoF, the better. Even with my Olympus I find I'm often stopping down my f/1.2 lenses to f/2.8 or smaller. Sure, I shoot them wide open a fair bit too, but that's usually more for lighting. I stop down the Fujinon lenses, too, but the lack of image stabilization requiring higher shutter speeds, and the need for smaller F-stops to achieve roughly the same greater depth of field, means the ISO creeps up very quickly on the Fujinon. It's a good thing that the high ISO performance is so impressive, but it means that the practical advantage over the Olympus is minimal.
Granted, I do still have poor discipline with DoF. I'll shoot the Fujinon 110mm f/2.0 wide open quite often, usually regretting that I didn't stop down when I review the images on my computer... but it's fun. You can actually get a more shallow depth of field with your Sony given the current lens offerings available on both systems, if that's the goal.
While I won't say that there aren't differences between my µ4/3 and medium format systems, the difference is not worth the $3,000 that divides the two. I worried that I'd never be able to shoot with my Olympus again after picking up the Fuji, and the first time I looked at the Fuji's images and compared them with my Olympus - not head-to-head comparisons, mind you, just reviewing my photos - it really seemed like I'd need to sell all of my µ4/3 gear. Yet when I mixed shooting with the two and reviewed the photos in a blinded manner, despite pixel-peeping and using my 27" retina iMac's display, I had a very hard time telling which camera took which photo.
More than anything, I think it means that there are no "bad" cameras these days. I know the Olympus very well, and know how to work around its shortcomings. I understand the company's design philosophy and feel comfortable with it. The Fuji legitimately can't do some things that the Olympus can (low-light exposure modes are an obvious example - the Olympus decreases the refresh rate to allow you to "see" in the dark, works well for manually focusing in very dim conditions, whereas the Fuji outright lacks that feature), but the larger sensor has some nice benefits.
If my rambling wasn't worth much on its own, here's the answer to your question: if you told me I had to go professional tomorrow, there's a good chance I'd sell all of my current camera gear and go with Sony's "full frame" mirrorless options. Their sensors are second to none for that platform, they have camera bodies geared toward specific applications, and their lens lineup is pretty versatile, even better with strong third-party support. The GFX system can be a nice differentiator and a possible selling point, but the lens lineup is still being established and the number of applications that the camera system can be applied toward is far more limited. The GFX 100, with its IBIS and on-sensor PDAF, rectifies a lot of that... but it's also $10,000. Unless you're shooting for clients who really care about the camera system or can somehow make camera gear a selling point for yourself, I'm not sure that you'll ever truly be able to justify the GFX system.
If you're just an enthusiast like me, then things are a bit different. If you've shot with it and truly feel there's a difference, and can live with the limitations compared with Sony, then you can justify it when you can comfortably afford it. Even though I may sound down on my GFX 50S, I enjoy using it and I love the photos I get from it, so it was worth it. Sure, the Olympus is what I take when I don't know what I'm up against and really want to ensure I come away with something usable, and I'd sell the GFX if my wife told me to choose only one, but this is a hobby for me and we only live once, right?
