the latest software does not fit on our device's internal memory

I’m not familiar with the “20” but usually you can add a microSD card in the compartment where the battery is installed.  I would check the manual for your specific device.  Carefully check and know the minimum  and maximum size to the card before you purchase.

It's a watch. You can't remove the battery and it doesn't have an SD slot.

Is it an Approach S20 OR WHAT?  Places do sell replacement batteries for the S20 and there are online instructions on how to replace it.  See for instancehttps://newpower99.com/products/garmin-approach-s20-battery-replacement-kit-with-battery-installation-tools-and-full-one-year-warranty.  But if it is an S20 you may be experiencing memory issues due to too many saved scorecards or map data, which can prevent software updates or course downloads. 

To replace the battery the glued screen has to be removed... Not a thing I would call "user replaceable".

1. All depends on the “user”.  As a mechanical engineer I figure somone built it, and if no special tools are required I can propbably take it apart.  I have replaced batteries and screens in iPads for instance.  Apple would like you to think it has to go to a repair facility but the reality is you need some plastic tools (to avoid scrathing the screen) and a hair dryer to break the adhesive bond on the iPad screen.

The majority of the users are not mechanical engineers.

Even if you're able to replace te battery the Approach S20 still doesn't have an SD slot, so you can't add an SD card...