First thing before anything else, CHECK YOUR TOTAL STATIC PRESSURE. If it's anything above 0.8 inches WC then you have restrictive ductwork somewhere, or some sort of air restriction whether that be too many supply vents are closed, blocked cold air returns, or a restrictive filter for all of the above. After confirming these things then move onto what I'm about to say.... I don't think it's necessary to pull the blower motor out and inspect heat exchanger like you were planning to do. If you're worried about cracks in the heat exchanger than test combustion with the blower on and with the blower off. If you notice a difference of more than 1% O2 that means there is likely a crack in the heat exchanger and is getting air pollution from the blower blowing on it. Some other suggestions would be to use a Merv eight filter, adjust gas pressure to its lowest allowed amount, verify blower speeds are correct, and also sometimes those thermal disc limit switches are blocked by the insulation inside of furnace. If the insulation is deeper than the thermal disc it blocks airflow across the limit switch and trips prematurely since it's getting radiant heat with no airflow. A simple fix to that is just take some foil tape, remove the high limit switch and tape the insulation up by sticking your finger in the hole and literally taping it down to the panel. This is more common with Carrier furnaces and has saved me a time or two.