You're over thinking this. Express will use the card if it needs to, but it won't tell you one is installed. I've used Garmins for many years and it really isn't an issue.
Voice files will not install to SD card but map files will.
If you want to convince yourself that the card is being recognized, change the DriveSmart from MTP mode to mass storage mode (just Google it and you will find out how). Now when you plug your Garmin into the computer, it should show up as two drive letters ... one for internal storage and one for the SD card.
After doing this, if you want to further prove that the card is recognized, create folder \POI on the card, run Garmin's POI Loader and install some custom POI's into that folder. Now when you start up the device, you should be able to find the custom POI's under Where To > Categories > Custom POI's.
I have a 3597. It was the first Garmin GPS I liked and am keeping it. It does what i need it to do. I too was struggling with Micro SD card. This morning I did reformat it to FAT32 and did a full format vs a quick one. It is a Samsung 16GB left over out of a camera. Turned of GPS and put the card in until it clicked. I used Garmin Express and saw that the GPS was 89% full and did not use the Micro SD card. I decided to reload the map to see if it would utilize it and it did. When I started to reload the map it asked me if it should use the Micro SD and then started to use it. So I think too that that card will be used once the GPS needs is.