Just hit MR15, but I still feel like I'm new

There's still so much to the game that I'm still learning, even though I've sunk nearly 600 hours in it thus far. I've been wondering if there's anything I need to be doing specifically? Like, I know the story missions are still my biggest priority right now, but when I'm not doing the story, I'm cracking whatever relics I can find for more frames and weapons, selling prime parts I don't need, and using the plat for mods or for slots for even more weapons and frames.

It feels like there's more I could be doing, but I'm not sure what?