Not sure where to go...

[Iscah]

So, I bought the latest story skip last night, and I'm now able to access level 80 mobs.

There's no real value to having access to "level 80 mobs" for their own sake, though.

Either you play level 80 group content, in which case you don't need to level further, or you level yourself to 90 in the level 80 zone and still only have access to level 80 content.

If you want to do level 90 content, you need to complete the level 80-to-90 MSQ by following Endwalker's story, and you will level up to 90 just by doing those quests.

Also, what is your actual final goal in the game if you don't like forced grouping? Do you have friends to play with? Again, if you want to join them doing level 90 content then MSQ is the only way to get there.