I'd also suggest doing the battery before anything else - I got my replacement Bosch from Tayna -
https://www.tayna.co.uk/car-batteries/types/096/
I needed a 096 AGM because I have start stop, I think yours is pre-start stop. Double check your battery number but I'd suspect it's a 096 like mine.
If it continues to lose charge with a new battery you might find the alarm siren batteries are causing that.