If you are running now on 24v, then stick with that, and make up the 19 ah 24v battery.
Series first, or parallel first won't matter, so long as the wiring is easy for you. i'd be inclined to make two 24v batteries by series first, then run a wire from each to the controller, with a Y connection ( paralell) just before you connect to the controller. Carry one battery on one side pannier, the other on the other side.
With that big a battery, 19 ah, you should be able to discharge a bit more than half, say about 12 ah. The larger a lead battery gets, the less it suffers from that use only 50% effect. You still want to stop at about 75% though, to get a year out of the batteries.
Better than run each battery separate, at 9-10 ah, they are suffering like a dog. Next spring, get your lithium. 10 ah big enough for lithium, but 15-20 ah better!