Reading your post, you said you cleaned it out because of soot. No need to do that. The gunk on the sides helps to seal the seams and reduces air leakage which will make it run hotter. I just clean the grills and water pan. I use water because in warmer climates my first 100*F are free during the Summer. You could also start with fewer lit coals bringing the temp up slower. Also, wind and even sunlight can affect the temp of a WSM. My lid thermostat runs about like your. You have meat between the Maverick probe and the lid.
Good luck with it. They are great smokers.
Weber WSM, Weber GA, Weber Spirit S-320
Charbroil Oilless Turkey Fryer
Wife LuzziAnn 1956 model.
Maverick ET-732, Superfast LSU Purple Thermapen
The Fun Never Stops at the BBQ Brethren Throwdowns
2013 Throwdown Grand Master. 1/2 of Team Paradise.