Congrats on your WSM. I just picked up a used (once?) 22 yesterday. I've had an 18 for a couple years and also have a mini.
I had similar problem with infiltration on the 18 when it was new. After a couple smokes that had higher temps than I wanted and with all bottom vents closed, I put some high temp silicone along the bottom seal on the mid section:
That helped to bring temperatures under control for me and I plan to do that on the 22 as well. What Harbormaster said about out of round is also important. Try to determine where the widest point on the bottom is and match that up with the widest point on the middle. Probably help to do the same with the lid as well. Some will try to get the sections perfectly round with gentle persuasion and that's probably a good strategy if you can get it right.