Another thing you can look at, probably alongside rather than instead of mass-gathering ores and things, is crafting materials needed in small amounts for alchemy and cooking recipes, like specific plants.

You can sell single items with a higher price tag than stacks, and help out crafters while you do it.

Another mass gathering item that always sells is Alumen (and higher-leveled versions like Black Alumen) which is needed for crafting all varieties of leather from animal skins.