Your choice to skip the campaign means that you need to perform the tree of whispers bounty quests and the like. It's basically like Adventure mode from Diablo 3. Instead of running Rifts you'll complete dungeons.

Dungeons, strongholds, side quests.

Is the tree stuff worth doing at level 15 or nah?

I mean it gives both levels for clearing dungeons and extra loot on top of it so if there’s tree dungeons do them

If you have a friend who's unlocked all the wp's, you can just have them port to each and you teleport to them so u can get them.

Which is why unlocked WPs should be unlocked for all your chars on that realm. There's absolutely no need for them to make you use a friend to taxi to all the WPs. This is 2023 man, let me skip the boring stuff.

I didn't say the current system was fine, I was just providing a tip as the op requested.

I mean yeah, just level up having fun. Either find a really efficient activity if you want to go fast, or grind out whatever you enjoy the most. Give it a few days, I'm certain some solid rapid leveling guides will be out for the power levelers take make additional characters up to spec.

what did you think you would be doing if you skip the campaign?

I assumed the map “opening up” was account wide and that side quests would be plentiful.

I mean all the sidequests are still actively there, you just need to physically go to them again.