A few of us boxers do M+ which can get you gear close to Mythic raid level tier from the weekly chest. The ones that come to the top of my head that have pushed quite hard content are Multidayz and SerScruffington.

I personally am the same, I box to have fun and challenge myself, how far can I get doing Mythic+? How geared can I get?

I also PvP, mostly World PvP as on my server at least it's quite active, but there is currently a mount called The Hivemind that allows you to multibox BGs, Multidayz also has a few videos on that.

Basically I'd say it comes down to 2 categories:
A) You want to make loads of gold farming the world drops and selling them or doing whatever else farmers do. (Sorry not too educated here as I don't farm).
B) You want to solo end game content, which in SL will be M+ and Torghast and maybe some other things I'm unfamiliar with. This is what I've been doing since returning to BFA and even though, in my opinion, BFA is hands down the worst expansion for me (I quit very early on when compared to every other expac), I have been kept entertained for literally months at this point, I think I subbed 4 months ago now? My Druids are ilvl 460 which still isn't that great, but I can feel the power gain from them and it's extremely satisfying to complete those higher M+ keys. But like I said, that's if you're more wanting a challenge from it.

I'll also say for me personally, another aspect is trying to help push boxing forward. By this I mean trying to discover new team comps that are fun, or maybe some new classes that can PvP (I recently found out today that Shadow Priest's Mind Sear ability allows them to face and interact with the target which most casters cannot do).

But yeah, honestly those are the 2 most common categories I see, either farming or pushing higher end game content.