How is a Thermistor supposed to be installed between Heated Bed and the Glass?
Try to use a silicone heat conductive pad between heater and glass, where you can make a hole for the sensor. It also helps to improve the contact between aluminium heater and glass, which otherwise may be inhomogeneous (as expected between two quite rigid materials).

They sell them in rolls and you can choose a 1 mm thick one or even 0.5 mm. They are "sticky" but they don't have glue, so you will be able to separate the glass from the pad, even if not so easily as it would be with a clip.