Run em until you get the text that the ruins have been 'comprehensively combed over' on hover then close them
Don't leave anything at the colony so you can close it without having to visit (unless its vast, then for sure put an alpha core + story point it for 50% extra loot)
Don't bother growing the colony past size 3 or building defences
Soak some mismanagement penalties and operate a couple extra tech mines at a time for more finds. Your tech mines only need 1 stability to not decivilize, the finds are not affected.
Make sure you make a note somewhere so you don't accidentally try to tech mine the same planet twice (never tried actually, but maybe you could rename the planet before abandoning it?)
This is how I found most of the domain tech items in my current game. Can't really do it if you've already started the colony game, sadly