You are welcome.
To explain a bit more in depth:
Adventure Mode – This is sorta automatic. The main would need to reach level 70 and then finish the campaign in Valdrakken. Then on the alts, all you need to do is complete the quest Excuse the Mess (If my memory serves me right) in the Dragonscale Expedition Campaign even though it’s possible to skip this intro quest chain to get to the Dragon Isles by either Boat/Zeppelin/Portal in Org/SW, however, the quests in each zone will not be available (at all) including the interaction with the Scouting Map. The Scouting Map will give you the intro quest (breadcrumb) to get to any chosen zone.
The map should look like this (Wow_tnpc603dGk.png) after completing a few quests.
Yes in that image you can see explore achievements and treasure chests, these are getting added as quickly as possible, we need to catch up.
As I have stated before, it would be best to hang on the main character as long as possible including pushing up to level 20 Renown with all renown reps, in the update 9.0178 you can achieve this with the Dailies & Weekly guides and also the Renown Rep guides.
Dragonriding – Once you did this on your main, all of your alts will be able to have this unlocked. Also if your main collected all 48 of the Dragon Glyphs, all your alts will be able to enable the traits on the Dragonriding talent tree.
This is done by clicking the Dragon Isles Summary icon on the minimap and then clicking the View for DragonRiding (Skill & Unlocks) You don’t need to visit the DragonRiding Skill Trainer like you had to on your main. (See Wow_0NP3ZXwPyg.png image)
Renown – Once your main gets to Renown Level 10 with all of the renown reps, your alts would get this buff Word of a Worthy Ally I where your alt would get 100% additional rep with each of the Renown Factions while their renown is below rank 10.
Once your main gets to Renown Level 20 (Renown) with all renown reps, your alts would get this buff Word of a Worthy Ally II where your alts would get 100% additional rep with each of the Renown Factions while their renown is below rank 20 and 200% if below rank 10.
So basically you’d be better off to get to Renown Level 20 because you get 200% additional renown rep up to level 10 and then 100% additional renown rep up to level 20. This one would be harder because of the longer grind, doing Quests, World Quests, Dungeon Quests daily this shouldn’t take too long. A lot of effort but it’s worth it if you want to push those alts through.
Remember, soon Blizzard will start looking at tinkering with the heirlooms so they can be optimized for Dragonflight. Which would mean, if your alts has some Heirlooms on, the gear during Dragonflight questing will defiantly replace those heirlooms in no time.
Here are a few youtube videos that would explain some of the above
https://www.youtube.com/watch?v=FNHUUjbxh0M
https://www.youtube.com/watch?v=O8vP36ambNI