I've been riding the Domane+ HP for a few months now. One of my purchase criteria for this e-bike was that I could add an additional battery. The reason for this was so I could do 100 mile rides. Now I come to realize that people are just carrying a second tube battery in a backpack or hanging from a top tube bag which probably now I would consider that for the few times I need longer range. I think you should be considering if you can do a 100 mile ride with 2 battery packs, not one. At the point I would be doing 100 miles I would probably rely even more on a battery than a 50 mile ride. 50 miles with maybe 3000 feet elevation gain using a combo of Eco and Tour settings is probably a reasonable expectation from a single 500w battery on this bike.
^^^^^^^"Now I come to realize that people are just carrying a second tube battery in a backpack or hanging from a top tube bag which probably now I would consider that for the few times I need longer range"
A top tube bag is perfect for carrying an extra battery. There is a 17ah HL battery in this bag that is did end up using on this ride some. To use it all I had to do was connect the lead to the motor lead via my anderson connectors. But that was because the cradles were incompatible. With the same batteries easy enough to just swap them out, or at least easy enough on most internal battery bikes. The bike, although heavy, did not feel top heavy at all and handled really well on the 50 miles of various grade logging roads .
In fact now I have a tt bag on my bikes full time just because. A light jacket and gloves fits easily with my pump and a water bottle on shorter rides and the small pocket is great for snacks, tool and the battery key. My preferred brand is Oveja Negra btw.
I personally don't like to carry any more weight on my back than a camel back and then only when necessary like on long rides, which is when I need the extra battery.
There is a 100miler that goes from where I live on the coast of OR to PDX via primarily logging roads that I hope to get to this spring and personally I don't see anything wrong with carrying a spare battery for such a mission.
One of my riding buddies inland has a +HP I just found out and looking forward to some rides with him.