I use BaseCamp because it is what I know with my Garmin Montana 600. There are equivalents to mange your way points, tracks and routes. I don't think Garmin Connect will work with the Etrex. As for map install it has been awhile since I have done this on my Montana since I have both Topo and City Navigator NA. But I have use OpenMaps. Then used Garmin MapInstall for installation of Specific maps on my Montana.

https://garmin.opentopomap.org/
https://openmtbmap.org/
https://www.gpsfiledepot.com/

See here for helpful tool to manage map files and backup a GPS.
https://www.javawa.nl/zumocolors.html
https://www.javawa.nl/

Good luck.

Edit. Get a cable...................