Just wanted to throw my 2 cents in since I work at an agricultural technology company that specializes in drones.
I think you've got a pretty sweet setup. We haven't messed with Autel drones yet, but they've definitely piqued our interest. A dual cam setup like that is definitely what I would have recommended for this kind of endeavor, although I would have probably gone with a DJI Matrice 210. I've actually used my personal Phantom 4 to help look for a downed deer before. While it could see down through the trees surprisingly well, I never managed to see the deer with the standard RGB camera. A thermal imaging camera would be a definite benefit. The only problem I see, like you said, is how long the thermal camera will be able to detect a downed deer after it dies. The deer's entire body doesn't have to cool to the surrounding temperature, only its outermost surface.