The above post is correct. You can get more fuel with the versions of trials bikes that are designed for general off road riding. The more you want to entertain yourself with the bike the more fuel you need, the more you want to watch wildlife the more you need a quiet bike. I agree with the above that the electric bikes would be very useful for a "stealth" mode to observe wildlife. More of a pain if you run out of battery than run out of petrol of course.
You are looking at two different needs and you will not find anything that does both well, just a bike that 'sort of' does both but is not good at either.
I have just bought an electric bike for the wife and if you do not ride it hard or fast the battery life is very impressive. They (Electric Motion) do a version with a seat (and larger battery). If you keep the speed down it makes very very little noise at all. https://www.electric-motion.fr/en/models/escape
Not cheap though.
Any modern trials bike will be great fun. You probably need to take some spare fuel in a can and refill it if you are riding any length of time. Depending on how young and fit you are your body will give up before the fuel runs out ☺️ (Mine does)
Look at smaller engined bikes too. My 250 is a lot easier of fuel than my old 280. The 125cc machines would be better and are more than enough to just have fun on.