I think what you've suggested is the most straightforward way to get the GPX files from your phone to the Garmin

You could use a file manager instead (if Garmin devices still show up as a removable disk when connected via USB) but that's just as fiddly - however it may be possible to automate the transfer using some kind of storage sync app.

As a side note - I was considering getting a Garmin to replace my older bike computer (which got stolen sadly) but the unit cost and need to convert OSM cycle maps put me off, ended up settling on planning routes and recording rides in OsmAnd