I suspect there's a way to increase the amount of structure space on planets. What's the secret? Also, how can I lengthen the amount of time my scanners are Good before they start returning Poor results? Other than upgrading? I'm only getting 3 or 4 before it degrades on me.

To increase the amount of structure space on a planet you need to use Quasi-Spacial Expanders Artifact. To get them ether hope for the 1 in 10k odds of finding the NPC that has a low chance of dropping 1 or buy them for 50 GP each.
There are other rare Artifacts that can be used on specific planet types but the change the planet type after.

Use Artifacts to up your scan and use Star Chart Purgers to remove bad planets from scans.
On a side note the good and poor rating when doing scans is just your odds of finding a planet , not a good or bad planet.

there is an artifact, the quasi spacial expander, that can be bought w/ gp, or dropped from a ship (i forget which one). it increases planet size, but you want to save 10, because they are needed to make a mission dyson for you, a collosal, unstealable planet with great resource stats. it comes after the terra project, which makes a mission exotic which is also an unstealable, awesome planet, and for that you need 10 terraformers, which can be bought w/ gp, dropped from a dark scrap carrier, or dropped from the same npc that drops the qse

Terraformers drop from T.O. Carriers I thought. I've only gotten 1 so far and the npc had "t.O." in it's name

i know its TO something. ill check the wiki later. and in responce to your second question, there are four stackable artifacts that increase scanning: tachyon cylinders (increase by 50), subspace scanners (increase by 175), continuum beacons (increase by 15%) and stryll seeker (increase by 10%). the 1st 3 are available from artifact draws, the last is dropped by the stryll hunter, skirmisher, and devestator. all effects wear off. oh, and the quasi spacial expander is dropped by the T. O. Troop Carrier (as is the terraformer)