Yea figured it would be pretty hard to find one, I actually was able to once get a ice world to get a breathable atmosphere. It started off as low toxicity then there was a colonization event that added 10 percent oxygen.
To get Boreas class world with breathable atmosphere:
1) It needs to be at least medium sized, the small ones will have carbon dioxide based atmosphere that is poison to humans and plants. Not sure where the exact cutoff is, but 5000km plus radius and 0.6g plus gravity should be more than enough. But 4500km and 0.5 g will probably not cut it.
2) The temperature on equator needs to be above zero. Even +1c is sufficient, it seems. Pick a planet with temperature at minimum around -10c. Fast roll is preferable for high temperature on tropics. Rerolling the geology part until you get modest atmosphere will also help push you past zero degrees (modest atmosphere doesn't seem to affect the composition).
3) It doesn't need a lot of water, even "desert planet, without surface water" is enough.
4) The atmosphere should not have so much carbon dioxide that it's outright toxic to plants, but minor toxicity is acceptable and will be reduced to nontoxic by the oxygenation event. It must not be insidious to humans due to too much sulfur dioxide (SO2), but small trace amounts are still ok. Look for atmosphere with RHL level at "toxic", it's there due to too much CO2 and will go nontoxic with the oxygenation event.
All being said, it doesn't really play that differently from any other planet, but it does have the novelty of being the only breathable planet type aside Siwa class. Some open farming might be possible on the tropics depending on how hot you got it (can be over +20c).
< Message edited by Antediluvian_Monster -- 8/8/2020 7:15:35 PM >