You must be using a low assist level to get 100km out of every charge especially if using the bike off-road. That's 5 watt hours per kilometre. I saw on a thread on endless sphere where someone was claiming to have got I think 6 watts per kilometre using a geared hub motor with the clutch disabled so could enable regen and that was road only and allowed for regen recharging the battery and many were suspicious although the general rule of 10 watts per kilometre isn't that far away. Now that I think about it it could have been 6 watts per mile so only about 4 watts per kilometre.
The expected range of that Bosch battery is probably around 50km but off-road its sometimes halved. Also the Bosch motors that peak high like 85Nm have been shown to go up over 700W in power sometimes close to 800W despite their 250W rating for short periods of time. That clearly would increase battery consumption. You can't have high torque/high wattage and low battery consumption.
Is it a good bike for just turning the motor off for flat roads and downhill? It looks pretty chunky and weighty.
One thing to be careful with is the battery. I was reading the other day of someone who had a fault with their Bosch motor, I think around 5,000 miles or kilometres and it was out of warranty and the expected repair price was over £$1000 possibly a replacement motor which the person couldn't stomach at the time especially for a heavily used and worn bike so the bike was left in the shed/garage for over 7 months. Anyway the battery self-bricked. The BMS is programmed to fail if it senses it is disconnected from power. The batteries are highly proprietary, filled with glue and have special electronic handshaking so you cannot use third party batteries. They also prevent you re-using the BMS by being programmed to die if disconnected from power so you can't reuse them. This also happens if the battery discharges too far. You could also argue that the battery cells themselves would be damaged by such a self-discharge but they have the potential to be recovered but sadly the BMS prevents that, the whole pack is useless. The battery still may have the potential to be re-used with other motor systems. I think even Shimano nowadays allow third party batteries so the cells could be re-used with a universal BMS. It's impossible to buy Bosch compatible BMS boards as far as I understand though. Anyway as Bosch battery packs age and the cells become lower capacity and may not hold charge as long a good regime of charging the battery perhaps every 2 months when you aren't using the bike is probably wise. I seem to remember another Bosch owner bought a newer model with a different motor system so the older Bosch model got less use and then the battery bricked itself.
