As you have probably noticed, offset is not as "efficient" as verticle. But, it is "traditional".
I would measure the temp at the cook grate(s) as it makes not a rat's ass of difference anywhere else in the cooker.
Now, this is just personal observation, but you may be depending too much on technology and not enough on learning management of the fire. I have a sneaking feeling you need to spend a few learning sessions actually sitting there monitoring the and temps...you may need a bigger stack or a bigger vent at the fire box...with any decent fire you should easily maintain 250-275...I'm thinking you need a bigger air inlet. But, I'm sitting here arm chair quarterbacking. I've got a build of my own to get on with so thanks for the reminder. Again, that's a nice looking build.
Southern Brethren BBQ Competition Team
"It's all about getting paid!" - Myron Mixon
"I love being hated in my hometown!" - David Hair
KingFisher Gator Rotisserie cooker, WSM, Stainless 5 burner with IR gas grill, Turkey Fryer, Weber JD Commemorative grill, slightly used Bandera chamber for future use as a cold smoker, Masterbuilt 40" insulated ELECTRIC smoker