Cool mod! I hate managing ship delivery schedules. However, I'm struggling to make it work like I'd want it to.

Ships prefer to run 300 science (not even a full rocket) from Vulcanus to Nauvis, while Gleba is sitting there with 45 items waiting for a ship to deliver them.

The problem is that science is continuously consumed on Nauvis, so by the time the delivery completes, the ship decides to go pick up another 300 as they're produced.

Meanwhile I've got hundreds of basebuilding items requested on Gleba and none of my 4 ships are going there.

Maybe a better heuristic would be to prefer full rockets or something? Or prefer delivering to places that have a larger number of outstanding requests?

I realized that I could lower the number of ships per route to 1 or something, which would guarantee a ship per-planet. Is that the intended way to ensure ships are distributed among planets equally? Or do I just need a way larger fleet?

Great diagnosis - you've nailed it. Right now the fleet has no fairness between planets: it serves them in a fixed order each tick, and a planet that's constantly consuming something on a short route (your Vulcanus→Nauvis science) keeps grabbing ships the moment they free up, before the longer Gleba run ever gets one. Per-item priority won't help here - it only orders items within one planet, not across planets.

Lowering max-ships-per-route is a legit way to force distribution today (it stops all 4 ships piling onto the science loop) - it's just coarse, since it caps every route. A bigger fleet helps a bit but doesn't remove the bias.

The real fix is anti-starvation scheduling so no planet gets ignored, plus an option to skip sub-rocket loads so ships don't burn trips on 300 science. I'm going to add that in v1.8.0 - thanks for the detailed write-up, this is exactly the kind of case that's easy to miss. 🙂

GOOD NEWS EVERYONE!
v1.8.0 has been released with this fix.