Having a bigger battery is good for having it last longer because you are much less likely to run it down too far. This is very hard on the battery - much more than charging to 100%. I make sure I never go below 20% before charging. I do charge to 100% every few charge cycles to allow the BMS...
Most bike speedometers only update once per wheel revolution since they sense the magnet mounted on a spoke. The update rate is a function of how fast you are riding.
Each motor needs a separate controller so if you want two controllers you can power two motors. Then you have to combine the throttles and other sensors. Not a trivial task.
These batteries hate low temps both when discharging and when charging. Most electric cars have battery heaters to keep them warm in cold weather. I fly RC models with Lithium batteries and I first noticed this many years ago when flying in winter. I learned to keep my batteries warm before...
13 to 15 AH isn't all that big a range increase. Not sure if your bike can handle it but higher voltage at same amp hour might give you more range. I'm running a 52V 12AH battery on my Ebike. That is about a 45% increase in WH over a 36V pack.
One big problem is the cost of good quality Ebike batteries. You can buy a decent doner bike for less than what you pay for a good quality battey. And battery costs are not really coming down anytime soon. Demand for these batteries is going up which will keep the price up too.
One of those will work but you should have a fuse and a switch so that it is not draining your battery when you are not riding. I have one like that on my 48V golf cart.