Decrease the amount of light arriving but increase greenhouse gases
Earth would be about 30°C colder if it was at its blackbody temperature, i.e. received the same amount of light but didn't have a greenhouse effect. The core idea here is to decrease the amount of light arriving while increasing the amount of greenhouse gases to maintain your planet's temperature.
One option is to be like Venus and have double digit percentages of CO2 instead of a few hundred parts per million.
There are also several relatively non toxic greenhouse gases that are hugely more potent than CO2: methane (23x more potent), CFCs (1000x more potent) and SF6 (A whoppimg 20000 - 50000x!)(1).
If methane or CFCs or SF6 are present in significant (not enormous) quantities, you could increase the temperature of Earth hugely.
Now, the amount of heat absorbed/emitted by a planet is proportional to temperature to the 4th power; if you can increase the greenhouse effect by 40 degrees, then you could have the amount of sunlight reduce by 50% and still have the same temperature; increase it 60 degrees and you can have it reduce by 65%. 100 degrees allows 85%.
See e.g. 184
https://www.astro.indiana.edu/ala/PlanetTemp/index.html or other planet temperature calculators out there; this forum doubtless knows many.
Maybe mankind could deliberately set off a methane clathrate gun or produce ludicrous amounts of SF6 to compensate for some event that knocked the planet further away from the sun. <PERSON>'s orbit change suggestion would work nicely.
(1) SF6 is incredibly inert chemically but is very heavy so it builds up in the lungs of animals if present in any significant quantity, eventually choking them. Your fauna would need some way to expel it from their lungs, maybe l by totally displacing all the gas in their lungs when they breathe out, or by means of an enzyme that binds to it and transports it to the digestive tract. I assume CFCs would have the same problem. Methane won't; it's light.
Edit: I'm guessing that seasons and maybe polar-equator temperature differences get minimised by doing this. 591