Aftermarket alarm? or factory? Either way, a malfunctioning alarm that is still hooked up could very well be the cause of your depleted battery.
Replacing the battery, and not knowing what was causing it to be drawn down will only kill the new battery. I’d be eyeballing that alarm system VERY closely.
I know it’s still flashing that light on the dash when the cars off, just don’t know how to disconnect it or check it. And I’m not sure if it’s factory or not.
if this alarm has a starter interrupt, this could be adding to your issues too.
Find a mechanic that is good with vehicle electronics, and get their input on removing this alarm system. This may just fix all your current issues.