So I quit raiding this expansion release. However i'm interested in getting the new legendary metas. however; do you really have to complete the 5.1 legendary quest line to qualify for the new metas?
yes you have to complete all the quests (including quest of valor that takes at least 6 weeks) to get the meta.
and you'll have to do all the quests including the meta gem for the next stage, and the next stage, etc...