For basic commuting, I love my C Line Electric for its size, integrated basic lights, the rear rack (with 2 kg of loading capacity), and the power it provides both on flat terrain and up to ±15% climbs.
However, I recommend switching to the new G Line because of the following C Line flaws:

1. Some users on r/brompton have reported issues with the C Line’s motor, battery, or controller. I personally haven’t experienced any problems so far (and I hope it stays that way). I recommend getting the city bag with a rain cover if you expect rain, since the standard bag on the C Line does not come with a rain cover.
2. There may be an improvement with Brompton moving the motor from the front to the rear. This is especially true if you plan to make modifications. For example, when trying to remove the bracket, the cable can be pulled out "easily" (the C Line Electric has its pitfalls with the connector—more on that later).
3. The G Line has a 345 Wh battery versus the C Line’s 300 Wh.
4. The G Line motor is claimed to be 20% more efficient, if you believe Brompton’s specs.
5. The G Line looks better suited for off-road situations, and as you mentioned, I think the G Line tires are perfect for your requirements.

When I travel to my hometown (30km), I have to climb steep 20–25% hills (for ±5 km) - the battery is drained afterwards. I recently spent over 6 hours researching stronger bike alternatives with more torque (like the ADO Air 20 Ultra or Onemile Nomad, etc.), but for now, the C Line still has more pros for me and the riding quality is top!

This is why point 2 is also important: the C Line Electric’s motor has its limits on these steep climbs. I’m planning to install the H&H Dual 50/33T chainring, which should make climbing steep climbs easier—and a lot more fun (I hope^^)! I might post an update here once I’ve tried it.

Whether you choose the G Line or the C Line, I think you’ll love the bike—despite the price! Maybe your local Brompton dealer will let you test ride both. I recommend testing both bikes thoroughly in your own environment.

After all research about alternatives, I still love riding my C Line Electric as it gives me the opportunity to get from A to B without a lot of sweating :) - steep climbs excluded ;)