Do all the sagas (most. I avoid 3BC and Gianthold is not worth it) and collect the XP after returning to 20. With the 50% XP pot, ship buffs and your 5% item, you'll be about halfway through level 27. You can get back to cap before the XP pot wears out.
Yes this is always good advice and what I do, but even then, A) a 50% XP pot means you paid for it, and that shouldnt be assumed, and definitely shouldnt be assumed for the entire ETR process. With at least a 5% pot, or maybe a 20% you saved up from somewhere, that only takes you to 25-26, and then B) that leaves you playing without your full character for those last several levels (27-30 is just under half the whole grind), and also C) that locks you into only doing those legendary sagas, and forces you to do all of them, rather than just playing at cap for XP as you like.
Incidentally, doing 3BC and GH right after you ETR and claim your Leg sagas is the best way to start covering that gap, but if you dont like those quests every time around the hamster wheel, then that's kinda proving my point.
A system that lets you "bank" any Legendary XP you earn (like with the Fey event items) and then cash that in at 12M XP for an EPL would be far more flexible and fun - that was my suggestion before, and would be a good example of turning a vertical progression system into at least a more diagonal one
