Give trade routes more variety: When reaching a foreign planet with a freighter select whether you establish a conventional trade route that yields money or whether you get instead one of the resources the planet produces (providing there is at least one and perhaps dependent on diplomatic relations) in small amounts, e. g. 0.1 per turn, increasing over time.

Perhaps you could even trade for research (e. g. yield calculated with the same formula as money yield for conventional trade routes, only that produced research replaces produced wealth in that formula).

Even better (like requested elsewhere in the past already): do not give trade benefits per turn, but let mini freighters travel the trade routes and give benefit when those arrive at their respective destination planets. Those mini freighters can be attacked and destroyed or raided (or both) by enemies and pirates. To counter that you can assign escorts to those mini freigthers (which are then taken from the fleet orbiting the planet, by some rule to minimize micromanagement, as long as ships are present). You can adjust how often mini freighters start. That doesn't change yield of the trade route, but balances risk (more freigthers means more regular income and less loss if one gets caught, but needs more escorts).