No problem with using AGM battery on the E53. There are some pretty good threads on here about some of that, it might be interesting to check those out. I have had both my E53 and my wife's E83 switched over for a long time and they have been great. I go out of my way to use Delco batteries though and have been for more than 40 years!
I'm not a big fan of maintainers and they are known to be hard on batteries, maybe some more or less than others, but if you can avoid them and take the draw off the battery it will for sure last longer.