All newer cars especially bmw have a volt meter on the + or - post clamp, when this meter reads under 14.4 like you said after running it triggers the battery light on your dash. The light is a charging circuit issue almost always ends up being alternator replaced unless broken/ chafed wires
The issue you are referencing is called parasitic draw, in your cause it seems to be your aftermarket Bluetooth. Car battery’s are not designed to be drained and charged. Marine car batteries are designed to be drained and charged over and over and not lose capacity. I would get a battery load test if you want to verify it’s the issue. Most shops can do this test but your battery may be under your seat or in the trunk just a bmw thing sadly catching on with other brands. So additional labor to get the equipment on the battery for the test. Also battery’s only last on average 3-5 years and seeing you live more north than me you have cold cold mornings that battery really don’t appreciate 😂 hope this helps
And to touch on the issue you asked between the battery’s you have to use the one that fits your car battery’s location. The length and width of the battery’s are different. The length width and height are your group size . To make this easy plug your vin into a auto website you trust and it will tell you what the oem requirements