DEM [Digital Elevation Modelling] is a quite generic term and also covers digital terrain modelling and digital surface modelling for example as well as physical 3D models, all entirely different to simple shaded relief. Garmin's usage of DEM in two descriptors for both the particular shaded relief feature of modern basemaps [gmapbmap.img] and the dedicated specialised DEM [gmapdem.img] for '3D Terrain' on certain units is quite correct albeit somewhat confusing if you're expecting that they display exactly the same. Both those uses result in a shaded map display roughly depicting the terrain in colour, but the basemap's DEM is very 'low resolution' and for that reason is only viewable when the display is zoomed out [it simply wouldn't be usable zoomed in anyway]. The DEM can be removed from a basemap and it will still function as a basemap, but if the basemap is not present on the unit or corrupt the unit may crash under certain conditions. The 3D Terrain DEM map is a type of supplementary detail map which can be selected and deselected on the unit in MyMaps page. Even if gmapdem.img is removed from the unit it will still function normally in every other way.

There are many reasons why a unit can crash. Most are related to incompatible, faulty or corrupt files on it [not only mapping files, but also Voice, Text, etc, etc]. If a crash occurs suddenly [when invoking Voice Command for instance], then it may be easier to identify the problem file. In your case, it seems that clearing nv/resetting results in normal running for a period before the unit becomes laggy before finally crashing. Whatever the reason, trial and error troubleshooting by removing or replacing the most likely files one by one to eliminate them as the problem is the best approach imo. Or you can do it the other way around and remove everything and add only the absolute necessities and test while adding files. I'd be following Caty's suggestion regarding a test after removing that travel guide for sure first though, then post screenshots of all current files and folders if you wish.