Here's my notes on this. I saw different posts covering parts of this, but I want to try and capture them all here for some traveler out there.

Before you End the Expedition:

• Note that you've accumulated a lot of quicksilver currency. You can use this at the vendor near the floating mission computer to unlock cool stuff. Quicksilver is the hardest currency to find, so you don't want to waste it. Any items you unlock with Quicksilver, can be reclaimed at the same vendor freely on any other save file.
• Note that each tier of the Omega expedition has an optional achievement for scanning aliens, minerals, and plants on each Rendezvous planet. If you missed these and want to finish them and you can't remember where those planets are, don't worry, all you need to do is visit the portal in a space station and view the previous space stations you've visited. You'll notice that some of these will have extra text on them like (Rendezvous #1). That will take you to the star where the Rendezvous planet is. Look for a lot of bases which are the purple icons you see on the planet surface from space. That should get you on the right planet.

And now assuming you really are ready to finish:

• Did you transfer important tech upgrades from your suit/multitool/ship?
  • Then make sure you transfer it back. Your main save does not have them now. You need to send them back!
• Transfer any other items back that you want to keep. If you're new to New Man's Sky, I'd suggest keeping rare eggs you received, Good S-Class upgrades you need, and especially any upgrade modules for your freighter (like +1 inventory to freighter, because freighter stuff is hard to find.
• When you arrive back in your main save, anything you stored back in the Expedition terminal on the Anomaly will be waiting for you, kind of like a locker.

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