Here's a whacky idea for epic tring:
1) Disconnect fate points from destinies altogether. Give fate point xp its own bar, like epic levels and destinies have their own bars. So purple epic level bar, yellow destiny bar, and green fate xp bar.
2) All unbonded destiny xp is wiped each epic tr, but fate xp is permanent, never to be wiped by any means.
3) You can only earn xp into 1 bar at a time. From level 20-28, you only earn epic level xp. Once (and not until) you max your epic level, you start earning destiny xp in your active destiny. Once (and not until) that destiny is maxed you start earning xp in your fate xp bar. If you switch your active destiny to an unmaxed destiny, you stop earning fate xp and start earning destiny xp for that destiny.
4) Epic tr would then require a) capping your epic levels, and b) capping at least one destiny. No xp is ever "wasted" because every xp you earn always goes into 1 of the 3 bars until you have the maximum fate points, which would be tens of millions of xp. Grinding out twists in off-destinies would become a tiny, trivial part of fate point grinding because you'd only have to do off destinies long enough to unlock the actual abilities you wanted to twist in.