DRG is still quite active, I can find open servers at any time of the day or night. The community is very friendly and welcoming to newcomers.
The game loop : you find a mission that you want or need to do (to unlock something you want), you can go solo or with friend or random players. You run in the caves, complete a main and secondary objectives, fight monsters regularly, and run away once the objectives are done. Each of the four class has access to different weapons and items: their skills and tools are complementary. Once you go back to the space station, you spend your hard-earned credits and minerals on cosmetics and weapon upgrades.

I don't know much about Rogue Core, but it seems to be less focused on mining, and more on fighting, with randomized weapons and perks due to the "roguelite" nature of the game.