Welcome to the community and congrats on the purchase! What speeds are you currently getting? We were seeing 25 mph unlocked and 20 mph locked w/ 165 lb / 75 kg rider on the flat for the 2019 WW.
I’d be a little surprised if slightly degraded battery would really be killing your top speed (especially right after fully charging) … unless it was seriously abused through something like improper storage or such. If you have a multimeter you can try measuring the battery voltage through the charging prongs at full charge and see what you get; if you do this though, you have to be extremely careful not to short the battery terminals during the measurement.
Both original WW and WW Pro batteries are 48V so you shouldn’t have to change the controller. I’m not sure if the WW Pro battery will fit, but maybe one of the other experts on the forum can chime in. @DrScootLittle
@Nordium is a battery expert and might have some more insights.