What is the drawback of a high-value resistor for positive comparator feedback / hysteresis?
1 M&Omega; for the positive feedback should be fine in your case from a impedance point of view.  Whether it always provides enough positive feedback under all conditions is something you have to decide.

What bothers me about your circuit is that the positive feedback is fighting against C23.  That is going to slow it down, which probably in part defeats its purpose.  To fix this, you can add some resistance between C23 and the + input.  The immediate effect of the positive feedback will be based on the feedback resistance and this deliberate resistance.  In the long term, the 5 k&Omega; impedance of the power supply divider will be added to the deliberate series resistance.