Currently bosch chargers a slow charge. So until they release a fast charger charging mid ride will only give you minimal extra juice. If I was you I'd look at upgrading to a bike with 800wh battery ability to add a 200wh extender. Have that capability with and hope the fast charger comes out sooner and yourl be sorted.
I also wish for a fast charge.
Sometimes I go for long rides (+70km), and normally I stop at restaurants where they normally help me in keeping the bike secured and even charge it (if I had the charger) - usually very nice people!
But @4A, even if it could be charged at 100% efficiency, 1h would equate to only 144Wh (<20% - 750Wh battery).
I hope I'm wrong here, but I don't think Bosch will push high current chargers with current smart batteries. Next probable/logical evolution, should be 48V systems, and even if they go thar route, I'm not sure if Bosch will have fast charging (@12A/or more).
Let's hope, I'm wrong!
The BIG issue of carry other battery, is really the dead weight on your back!
Even if you start with a 500/625Wh (750Wh on your back pack), and you're still fresh, you'll carry home a +3kg back home at the end of the ride. It doesn't sound much, but it's HUGE weight at a very high location from CoG.
Beside that, on some bikes, the battery plugs to the ebike near the headtube, and shorter batteries will have spacers near the motor (to make batteries the same lenght has 750Wh).
Having this spacers near the motor, will translate in having the CoG (~½ the lenght) of smaller, lighter batteries up in the downtube, which will have a significant effect on how you feel the battery weight difference.
In such bikes the difference will be almost not noticeable, when you're manuallying or lifting the front end.
And to take full advantage of lower weight and lenght, means inverting the spacer position and assemble a new plug on the spacer [some users that adapted 400Wh batteries on their RIGs, have that well documented].
Either way, carrying an extra battery on the backpack is not that fun, and probably is better to position the 2nd battery at a cafe/restaurant and swap once reached, or position the car in such a way that mid ride, you swap the battery...