There is only a chance to receive those other items from salvaging, and the chance is very low. Whereas Shards and Gold are a guaranteed drop from salvaging your excess Gear, Spells and Minions.
As can you see in my example, there is a 0.8% chance this salvage could get me an Epic relic
Just because I’m likely to be missing something, how on earth do I obtain epic runes. All I seem to get are legendary runes (few of those as well), and I can’t progress because my weapon is stuck at 25 along with 70% of my amour.
It’s very frustrating and I was enjoying the game so much.
You are at the dreaded epic bottleneck. Sadly, epic runes are much more difficult to obtain than legendary or even mythic ones. The sweet spot seems to be in lvl 20-25 chests, but still not very high drop rate. The best way for getting epic runes is doing certain adventures. If you have Darkhunter follower at rare, then you will be able to craft them for 100 gems. Otherwise, you can buy them for 375 gems at the shop (each day has runes of certain color).
You can always start a new playthrough of the game with different heros (and maybe the same gear). This way you get all the first time bonuses again from the story and the follower missions. Hopefully while you’re doing this you get enough runes to upgrade the main gear that you would like to upgrade.