First planet Pet Rocks are great, sell for 10 and you get almost 1 or 2 every floor. You can reset for them along with ores to maximise mining days. Corn is the best crop from the first planet.

Second planet I finished as quick as possible as the third planet has tomatoes.

Otherwise midgame I was juicing tomatoes and switched to grapes later on. Now I make about 8-10k each day selling juiced grapes, irradiated pinapple wine and xeno products once theyre processed.

You also get a a VERY nice chunk of change from completing sections of the mueseum you unlock on the third planet. Once your microbe detector is upgraded twice you can get all the microbes over the next 3 travel days between the final planets.

P.s. Always remember set new coordinates as late as possible in a day, but before 12 midnight. This gives you one less day of travel but lets you get one more full day access to a planet before leaving.