Same thing here. I went with Sony because of how expansive the E-mount lineup was, including 3rd parties (even the lesser known ones like Samyang and Viltrox).
At the time, Canon’s first party lineup was pretty lackluster if you weren’t looking for the L-series professional lenses. It’s gotten better since then, but in terms of value it’s very hard to compete with Sony since third parties are much cheaper, and used availability is much better due to the age of the system.
I’d also recommend seeing how the USB protocol for your camera works. On mine (Sony A7R III) there are some relatively impactful limitations. One is that you seemingly can’t shoot a burst directly over USB, but you can work around this with a shutter release cable. The other is that you can’t change shutter speeds and ISOs super quickly, you can only increment/decrement them. The latter issue is fixed on newer models though.
I haven’t done this exact type of thing but my understanding is that the radios and wireless chipsets of most cameras are pretty poor in comparison to what you can get with a phone or computer. USB tethering is probably the way to go to reduce bottlenecks. You could potentially tether to a phone (though idk what the software situation looks like) or possibly an ARM SBC, as some have LTE and 5g compatibility.
I don’t have experience with the Fuji system but I would make sure you’re appropriately budgeting for lenses. The common saying is “date the body, marry the lens,” meaning spend more on getting good lenses than the body. You can hold onto good lenses for a long time and upgrade the body as needed, and good lenses can get super expensive, especially in wildlife.
The kit lens won’t give you enough reach for most wildlife, and it’s probably not the sharpest either.
I’ve used eBay, /r/photomarket, and MPB for buying second hand gear. If you’re using any BST platform like photomarket, make sure you use a payment method with buyer protection, and regardless I’d make sure to fully test and inspect the lens upon receiving it.
A friend of mine has bought cameras and lenses off Facebook Marketplace, but sorting through the scams to find a real listing can be really annoying. I’ve never bothered with it myself.