To facilitate faster, cleaner, and easier changes, it helps to install a drain plug in the trans pan. I've done several on the A4XS/AXOD types and found this to be a good location:
This makes routine fluid and filter changes a lot more enjoyable project. I'll second the notion that changing the fluid and filter regularly is the best thing you can do to prolong the transmission. Use the original type EPDM/steel spacer gasket whenever possible. The replacement gaskets usually provided with a filter kit are not suitable, and are generally only a leak waiting to happen.
Chances are that most of the metal fines you are finding in the pan and on the magnet are from the chain and sprockets wearing slightly, and that is not a major point of concern.