Also don't worry about mixed runes that much. They are mostly used to min max a merc and you are usually better off just using pure runes.
Also a guide of what can be a consider a "good" rune when you upgrade it to +9 is as follows
Assault% rune= 45% atk and higher
Assault flat runes= 450 atk and higher
Vital% rune= 45% hp and higher
Vital flat rune= 1500 hp and higher
Shield runes= 25% def and higher
Rage runes= 125% crit dmg and higher
Fatal is anything that can get you to 100 crit rate with supporter buffs
You should only aim for these on epic and legend 6* runes. Rare runes shouldn't really be refined because you'll inevitably ditch them. You can also go for 5* runes as well if you them. The only 4* runes you should even upgrade are legend shield runes. They are the most likely to get that 25% if you are lucky enough to roll for it.