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You - or anyone else - might like to try this one (in the modern GMAP format).

http://www.the-thorns.org.uk/usgs/USGS_UT.zip

Made from recent USGS and USFS data with some items from Open Street Map. Also the Bing buildings data.

The Oregon device may have a 'Map detail level' control that could make a difference to what is displayed.

I changed from cGPSMapper to MKGMAP a couple of years ago.

It may not solve your error but it is much faster and free!

If you go to the link in my Avenza post, the shapefiles there include MVUM details as well as USFS Roads and trails.

I've used the shapefiles https://data.fs.usda.gov/geodata/edw/datasets.php?dsetCategory=transportation and merged them with USGS data to make some Garmin maps.
Email me if you want a link to Montana.
I can make the same map in Mapsforge that Orux will handle.

IMG file have a bit set that determines whether Basecamp can 'see' the map.[This is to prevent maps created thru Basecamp being seen double]. There is a tool called Javawa Device Manager that will change the visibility - worth trying.

If you want to preserve space on your system disk, copying a shortcut to the .gmap folder instead of the folder itself works well for me.

The KMZ tile limit (100/500) is per device but the JNX limit (50,000) is per map so the limit will be storage.

I don't recommend it but - for completeness of the discussion - there is a non-Garmin firmware patch that allows JNXs to be used without Birdseye.

The guy - Alex Whiter - who did much of the work on JNX and the patch died suddenly last year.

I have been using MKGMAP - which is still being supported and developed - with .mp files and it is much faster than CGPSMAPPER.

IMG files contain a bit that determines whether the map on the device is visible in Basecamp.

Maps installed with Basecamp are not usually visible because it assumes they will stay in Basecamp.  But if you've removed it from your desktop device then it will not be visible.

But there is a javawa utility (Device Manager, iirc) that allows you to make it visible again.

If you select track summary, GPSU itself reveals tools to simplify and compress tracks

Open the GPX in GPSU and click the map icon to draw the track.

Then File>Copy bitmap to put it on the clipboard

The simplest way, I think, would be to open the track in GPSU, save the image then use Photoshop to select the track by its colour and then paste that onto the background.

There are technical reasons why a single Garmin map of the US cannot be made. Garmin devices vary but are limited to a few thousand map segments and, for the maps I make from USGS, each state is a few hundred although it depends on the level of detail, too.

There is a utility software - GPSU - that interfaces with many types of GPS unit. Maybe post the question in their user forum. www.gpsu.co.uk