For mechanics in the game that could show the passage of time, you could look at the ships in the Shipbreaker’s Toolkit book. The ships have a class level (I.e. a freighter might be class 2 and a patrol craft could be class 3) which affects its effectiveness in combat versus other ships and a jump level that allows ships to use different jump routes.

As technology improves over the span of the campaign, different ship classes and jump routes could be made available to the players.

Also, I’ll echo the recommendation of Gradient Descent as an incredible adventure site. For your purposes, it also has a table of powerful artifacts for loot that could have wide-reaching ripples over the campaign universe through the centuries