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In general, I would extend the chimney down to grate level with some aluminum dryer ducting. Get a couple of wired probes, that therm is useless placed that high above the grate. Make sure you have a good coal basket in the firebox. After that, monitor temps from right to left at meat level on the grate. From there you should just have minor tinkering left to do.
I would get the other issues resolved with therms at the right and left of the pit before messing with a baffle. I found on my unit that careful placement of my drip pans can provide a baffle effect.