I would do it in the following order:
1. Purchase an i115 weapon from the Market board (not sure about this one as you still only have ARR); just the weapon, see 3 for gear.
2. Level alternate classes till required levels for cross class abilities of your main job.
3. Run your daily roulettes (as lv50) to get Poetics (i120 gear purchased in Mor Dhona). All lv50 quests off no experience any ways.
4. Start the Relic weapon story line. This will continue and evolve many times over while you are completing everything else listed after this. If you stick it out, you'll have an i135 weapon while will last you in Heavensward from 50 - 58.
5. Main Story Quests until the end of 2.55 when you begin your travel to Ishguard.
6. Complete each HM lv50 dungeon to unlock High Level Roulette.
7. Complete each HM primal
8. Complete the Hildibrand quest, by this time, it should unlock your Trials Roulette. FYI, 3 trials are locked behind this story line.
9. Fill in the rest here, Extreme Primals, Beast Tribes, Crystal Tower, other misc. quests, crafting, gathering, level up alternate classes.