Level all characters to 20/40 for the Fate.
From that point, getting them to 50/60 takes around 2 days of F2P resin budget.
This is worth overall, as there are no other ways to convert Resin to rolls. I would treat the 70/80 Fate as a bonus rather than the goal. It costs 5 days worth of resin, which is probably not worth the expected rewards from a single roll.