Let there be absolutely none, exactly 0 axial tilt. Let the earth's orbit around the sun be a perfect circle with 0 eccentricity and a surface to surface ( constant) radius equal to the harmonic mean of the surface to surface earth to sun distance through the past 800 years(an integral if time is thought to be continuous or measuring the distance every quantum, least increment of time, for 800 years). Let the sun be a perfect sphere and the earth be a perfect sphere(no mountains, cliffs, valeys, deeps) too. Let the angular speeds of earths movements(Revolution, Orbit, etc) be ever constant(a day of 24 hours and a Year of 360 days). Let the consistancy of the Earth to the core be perfectly uniform with a perfectly uniform surface(allowing the exception for an amount of rivers running through perfectly straight lines of longitudes with a symmetric distribution between the latitudes so the human civilisation could have water and thrive). Let the consistancy of the rivers/ocean and atmosphere be perfectly uniform with a breathable partial pressure and a tolerable total pressure.
I tried to make the climate as uniform as possible and not simply remove Milankovitch Cycles. I do not have any ideas how the climate could be more uniform if anyone has any ideas I would be happy to think about them.
Now to my question
How will Earth's climate be and how will that climate affect human life and civilisation?
I think that the poles will shrink as there would not be any winter, the variance would be minimal and largely due to longitude difference rather than to latitude differences (proximity to the rivers could play a minor roles), the average temperature would increase, as less heat will be radiated back to outer space and more of it kept on earth (Radiation of Black Body). I want my hypotheses confirmed and a way to imagine how the climate would be. As for the consequences to life and civilisation I have no ideas.
N.B I tried to ask a similar question on Earth.Sciences because I thought it belonged to climatology and planetary science but they refered me to worldbuilding.