Once you use it a few times and get it all sealed in, I find that the WSM is very good with temperature management - to the point of being boring. The main issue of a long cook is running low or even running out of charcoal in colder weather.
Why not try it out for a while before you pull the trigger on a temp controller?
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