Depends what you want and what you have.

It is very likely you can use a full relic to get to 70, at the very least up to 67 or so. It also has the usefulness of being able skip the first step of the next one (likely) and if you are truly "2/3 or so through the umbrite/crystal sand upgrade phase for the 240 level." the rest of the steps is 2-4 days total if you grind it out. In 5 days I had a friend that had all the items saved up for 2 relics, and in these 5 days (after .. the maybe.. month or 2 of dailies) did lights , dfs, etc for 2 relics.

The only little issue I see is you do not have the benefit of having 2 weeks for those quests for clusters. even just doing the 3 leveling for this week................ if you get the umbrite stage done over the weekend, you can do this week and next week's weekly for clusters to be 36, so I def recommend getting it done over the weekend so you can do the weekly. (since you would only need 14 after doing the weekly twice)