My Drive Prism Sport was bought brand new in 2015 so is now 4 years old.
It has never been serviced and I was going through batteries 2 x 12v 20 Amp various makes every 8 to 12 months.
Here is your problem.
1. your motor is less efficient than when new. Weaker magnets, less accurate running on bearings, worn commutator, worn brushes. But that doesent meant its completely worn out. It does mean you get less miles per Ah.
2. You are overworking the batteries. To high discharge level daily.
3. You are using 20Ah batteries where 50 or 70Ah batteries are really required. Sorry but thats the truth of things.
4. The biggest problem. You are using cheap generic AGM batteries. Those are JUNK, and they have both a short cycle life, and they are too high impedance or internal resistance. Meaning you see a big voltage drop under load. As you already found out.
I now have Leoch 12v 22 Amp batteries only 2 months old and they are great, but have the same problem with the scooter.
With a full overnight charge around 10 - 12 hrs (New charger too), when moving forward the analogue needle gauge drops from full to about 70%,
I thought it was because I'm 17 stone, I put my able bodied skinny friend who is 10.5 stone on it and it does the same.
Because the batteries are TOO HIGH IMPEDANCE (Resistance). So as you draw Amps, the voltage falls.
The real answer is Odyssey low impedance batteries. And a proper charger. And dont discharge as deeply every day, or get a proper sized scooter with 70Ah batteries. Or go lithium.