I believe your offset is made of pretty thin gauge metal which is difficult to keep good temps going at those outside temperatures.
Did you use the Minion method to light the coals? Do I understand you correctly that both the top vent and the door to the firebox were both fully open?
Jim - Another transplanted Texan
Former KCBS CBJ
Large and Medium Big Green Eggs , Black 18.5" WSM, Blue Weber Performer - Stainless, Green Weber OTG Kettle , Brinkmann SnP Pro, and a Stainless UDS. One retired Portable Kitchen grill.
Red Thermapen, Maverick ET-732, EdgePro Apex Sharpener.
Avatar is the original 1951 Weber Kettle