This ain't the Bascamp I used in the past...

I had a Nuvi 40 and it's very much an entry level device with limited abilities. It's older and slower with limited storage, and I think it's probably obsolete.  Meanwhile, maps have been getting larger and more detailed over the last few years.

BaseCamp is able to work with a wide range of Garmin devices, but it can't add facilities which aren't supported by the device.  Sorry.

As for your BaseCamp issues, I can only the suggest the "standard" techniques for handling any errant Mac application:

- Delete BaseCamp from your Applications folder and delete any downloaded installers.

- Rename its preference file (Library/Preferences/com.garmin.BaseCamp.plist) to change the ".plist" to (say) ".xxxplist"

- Download a fresh copy of the installer, and install it again.

I also suggest you download a copy of your map onto your computer so that BaseCamp can use it, rather than relying on the map installed on your Nuvi40 and connected by a USB cable.

The nuvi 40 is an old entry level device and isn't capable of using routes so it's not a BaseCamp issue. You can change what BaseCamp displays in options and turn off airports or simply change the overall detail setting to a lower setting, whichever works best for you. Map movement if you're using the map from your nuvi will be slower than using one installed to your computer. I doubt though that Express will allow you to install the map, to enable that option you need to update the map on your nuvi first, are you able to do so as many older devices came without lifetime map updates.

So I did as suggested but my issues multiply. Now, when I use Garmin Express on another Nuvi 40 belonging to a friend, its starts communicating, then shuts off the Nuvi, bang. No idea why. Only plists i see are

-rw------- 1 2 staff 24721 Jan 25 21:24 com.garmin.BaseCamp.plist
-rw------- 1 2 staff 17946 Jan 25 20:43 com.garmin.MapInstall.plist
-rw------- 1 2 staff 212 Jan 18 01:58 com.garmin.MapManager.plist
-rw------- 1 2 staff 286 Jan 25 21:29 com.garmin.renu.client.plist

I went to a WIN10 machine, installed Express, etc. on it, and successfully updated the maps on my friend's various Nuvi. Interestingly, some seem to have more storage space than others; they can take the full map vs, part of the US.

I'm not surprised that different Nuvi models have different amount of space.  However, a limiting factor I bumped into with my Drive 52 was that a system folder was getting filled with a number of huge files, some of them over many hundreds of megabytes.  Despite fitting an expansion memory card, Garmin Express couldn't install an update map.  I had to find and delete the oldest of these huge files to make 1 GB of free space ON THE DEVICE, and only then would the update take place.  The update loaded the map to expansion memory card, and loaded more huge files onto the device!  I guess I'll have to delete these files again next time I do a map update! 

So maybe the Nuvi 40's all have the same space. Given the unexplained connectivity issue with Express & my Mac, I didn't spend the time I should have looking for hidden files. In retrospect, I should have removed Express so I could use ordinary Unix tools on the mounted  Numi's.

I'm curious as to which system directory you found filled up? Or to put it differently, how did you know what was safe to delete, and what wasn't.