This stuff is doable.
I'm going to reiterate that I like Topofusion for working with trails. The "Network" tool is an automated method to take a bunch of tracks and make a trail system out of it. It's not perfect and you might have to edit some stuff to make the final product "right" but the learning curve is less steep for this than full GIS software like QGIS and others. I still use Topofusion when I record an activity with my old Oregon handheld because the active tracklog gets split up and dumped into archived bits that I have to piece back together before I load them to Garmin Connect or Strava or whatever. The process to piece these segments back together with Topofusion is much easier than other options, too.
So, take your data and make your "network" out of it. Once I have a "network" I prefer to get the file format out of .gpx to make sure that I'm distinguishing between file types so I know that this specific file cannot be loaded straight back into my Garmin. I use .shp files (a GIS format that's actually a collection of files that describe different spatial components of the data).
Unfortunately, Topofusion's tools to make new routes (that follow existing paths) isn't good. You
can use it to make a new .gpx file, but it's not going to "snap" to your trail network file that you made. You can create a new file and then draw out what you want, but it's going to just be freehand drawing. You get two operations, single clicking to draw straight segments, or click-and-hold to speed up and go more freehand (and sloppy). The GIS programs I use can build a new file and trace existing paths. ArcGIS doesn't support .gpx files natively, but has very good tracing tools. QGIS does support .gpx, but it's been awhile since I used its tools this way, so I can't say how it compares.
As for making this data routable on your GPS, I did a little digging to come up with the methods. First, a FULL tutorial on making custom maps for Garmin devices is here:
How To Create Garmin Topo Maps - GPSFileDepot
A big part of the tutorial is related to where to source various data (roads, streams, contour lines, points of interest, etc), data conversions, and various formatting issues. It's WAY more complicated than necessary if all you're trying to do is work with this single trail network file. But it can provide some insight about what's going on behind the scenes.
For taking your trails and making them routable maps on your Garmin, consider a program like this:
GPS mapping software Mapwel, GIS shp to Garmin GPS device
I haven't used it so I don't know the exact process. But it'll do the job. In cases where trail segments are connected by using some road segments, you might have to dig into the first tutorial I mentioned, depending on how complex the network is, how much of those roads you need to use, etc. If it's just short segments, then I'd just continue to trace/draw/connect the trails as though the connection was just a bit of trail. If the segments use lots of trail, then I'd consider adding road layers to the map.
Now, of course this doesn't help with planning a ride on your computer that you can then load into the device as a Course or whatever.
If you don't care about routing on your GPS, and you just want the trails to show up on the map so you can see them, then this program is probably easier:
Converting your gpx files to Garmin compatible IMG files :: GPX2IMG
