boro glass will take care of minor inconstancy in leveling. However if your pcb bed is warping bad (ie causing your boroglass to lean and not stay level.) that's another issue. remember your setting your glass on top of a surface that is not flat, and changes as temperature changes. If you dont want to change up to a kapton heater, just get rid of the pcb heater. level out the birch ply and place the boro on top of that. You will be limited to pla but at least you can print. something worth trying till you get a better heat bed.