It reads like the battery is, as you say, discharging near to empty and then recharging each time. Really you shouldn't leave a car for more than about two or three weeks before starting it as that'll kill the battery more quickly than keeping it at a good charge level.

It's likely you do need a new battery and they are relatively easy to fit yourself, however some owners report that the Battery Monitoring System (BMS) needs to be reset via official diagnostic software through an independent garage or dealer. Many owners said they didn't do this and no issues arose, however. If this is the case it's only the electric windows that may need resetting to position them. We can't find any reason why your battery can't be trickle charged, though.