Rarity

[MelodyCrystel]

[...] - Astrope (two-seater giant-pegasus-mount) can be earned by completing 2.000 mentor-roulettes - that requires that you have your mentor-status, though. Which in turn requires that you complete all 4 role-quests in Shadowbringers (tank, healer, physical-dps, magic-dps)[...]

You forget the real obstacle before starting the grind:
How to unlock Mentor-roulettes.

->One must have completed all dungeons / normal trials / 8-men-raids / 24 people-raids and all old Ex-trials (which is at the moment everything from ARR up to Stormblood) to have access to the roulette.

EDIT: Or, in short, you need to complete everything that isn't Savage-, Ultimate- or current Ex-content.