Isn’t there a “journey to the centre”

Yes, though it's a cheap "new game plus", without even feeling like a "new game". Once you manage to get to the center, you're thrown in a new galaxy in a random planet and have to get back to your ship, only some upgrades you've installed on yourself and your ship might break. Yet you can immediately call your freighter and any exocraft.

There's also Artemis' questline, with an interesting concept but overall underwhelming delivery.

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interaction with other players?

From my experience, which pretty much ended around 1 year ago with the game, player interaction could be summed up to:

• finding someone else's buildings during a community expedition
• finding someone else's buildings in a quicksilver quest
• someone giving you free stuff while you're idling in the anomaly

Apparently there are guilds now? In any case, I never saw anyone looking for group, because the game has nothing that only a pair or trio can do, or do faster/better than a solo player, other than base building

From what I've found online, the "center" of your galaxy is just a portal that sends you to the edge of a different galaxy where it's different but more of the same.

Kinda like Starfield actually.