Your desired hysteresis is 0.75V but your observed is 8V (implying a +/-10V swing at the output of the comparator U1A) so decrease R2 by a factor of 0.75/8 to 3.75K.
You want it centered at +2.875V so add a resistor from the non-inverting input of U1A to the negative supply. Value should be 12/2.875 * 3.75K or 15.65K
That gets you the desired swing and the duty cycle is unaffected, but the frequency will be different (about 10x higher) because the hysteresis is less, so increase the capacitor by the inverse ratio to R2 or 8/0.75 to give you 2.1uF or leave the cap the same and increase both R3 and R4 by that ratio.
And better to replace the 1N4xxx with 1N4148 or similar switching diodes.