Download offline maps per country

For overlanding I need to be able to download much larger files. Preferably per country. I cannot download maps along the route due to lack of fast internet. So, I need to download everything before leaving.  Lack of this possibility is a major disadvantage from Gaia GPS to Garmin. Considering buying a Garmin Tread Overlander XL with all European and African maps preinstalled on the device.