I have found that car batteries seldom hold to their warranty period. I believe my car sat many months in LA, CA. before I had it shipped north to Seattle. I've had it for about 15 months and in the last couple of winter months, noticed a sluggishness in starting up with our temps, at worst, 40F.
I suspect the CTEK you mention is a solar unit ?? Not useful in my garage but regardless, plan on hitting up BMW for a battery check and, hopefully a replacement. With the old batteries, you could usually add some water and give enough life to get a start when dead.
Currently, I'd be apt not to spend extra money on a premium battery but stay alert to the cranking power and have it checked prior to being stranded and have it swapped out for a new one paying the difference in warranty.
If I wasn't using the car for extended periods of time, I'd disconnect the battery. You may be able to leave one of the cables connected to keep memory settings intact but not drain the battery.