I have just joined the forums page on gpsfiledepot.
Your question is very similar to what I am researching. I am originally from New Zealand and have been living in Ireland for 16 years. I bought a Garmin GPSMAP 66st exactly like you and have been using it for the last 2-3 years. I find Garmin handheld a "God-sent" and it has navigated me off the mountains in zero visibility situations. I found the maps that come with the Garmin handheld very poor with little detail of hiking tracks in my area.
I find the maps from OpenTopoMaps org excellent for Ireland and UK hiking tracks. I learnt how to download OpenTopoMaps maps onto my Garmin for free. You can export your gpx.files from your Garmin handheld onto the online OpenTopoMaps and it shows you the elevation profile of the mountains you hiked. I use some excellent tutorials that helped me optimise the settings on the Garmin handheld and helped me learn to use it properly – from GPS Training UK It cost £50.00 for an annual membership. There are tutorails on using BaseCamp and Birdeye also.
I stumbled across the free QGIS software recently and I am teaching myself how to use it (slowly). I want to lean how to plot a track on a map via QGIS, convert to a gpx.file and then download onto my Garmin handheld.
Are there any useful tips you can share with me?