Seems to me you have no real reason to change systems or bodies. Just seems like GAS to me. Maybe I'm incorrect.
AF: A growing family cannot outrun the AF. The old 5Dc can track a running child, let alone a kid running bases at a ball park just fine. And the 5D2 has that same AF system. Sure, it's not super robust, but it's far and away fine for kids and human beings moving around, even in peewee or youth sports. Maybe how you're using it is not conducive to the AF system (like, are you trying to track a moving subject with an outer point in low light?).
ISO: You can reliably shoot ISO 6400 on that camera without much effort. Since you're an enthusiast, maybe look at how you're exposing with high ISO and in low light, and adjust how you expose so that you have more room in post. You can get great clean photos at ISO 6400 & even 12800 on the 5D2. Moving to a new camera, you either lose this (Fuji with a smaller sensor, will not have the same high ISO performance at ISo 12800) or you could gain less than 1 stop (5D3), which screams to me that an upgrade or downgrade has nothing to do with ISO unfortunately. You'd have to look at a camera like the Sony A7S if you wanted to see more than 1 stop of ISO difference in low light, for your $1k or more you'd spend.
Value over time: Everything depreciates. The 5D2 will not drop below $700 for a few more years. As long as this camera can be serviced by Canon, it will still be a highly traded Camera. Do not base a purchase on this. What's losing $50~150 over 2~4 years? Years of service on a camera for memories that are priceless. Stop worrying about a few dollars for this purpose. If you want to spend money or move systems, take your eye off the monetary value and put it on the features and results that you want and focus there instead.
I think you have a very good kit as it is. I don't see a reason to move to anything else at this point.
Moving to a Fuji system, it would be for size reduction. Not for quality increase.