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There is actually another kitchen related fix that works well. I place the patty on a sheet of waxed paper, the wooden guides under the waxed paper, and then another sheet over the top. Rolling it between two sheets of waxed paper keeps it from sticking to the rolling pin.

Also, it sounds like you either didn't get it wet enough, or have a poor binder. When trying to make a patty, I find I have to get the water content over 20%. If you have a frame or a loaf box you can use less.

I'll see if I can find the website I'm thinking of where they tested things like this. However you may be able to double or triple up sheets to meet your protective requirements.

Could you possibly describe the appearance of the lumber? That may provide some insight into what it is treated with. They used to treat wood with nasty things like arsenic in combination with copper and other things. I believe they've been moving toward treatments with things like boric acid and other copper salts. Copper is quite an effective agent to deter fungus and rot causing organisms. I doubt that the treatment agents would harm the char, or affect the final product too much. My main concern would be what else is coming off with the smoke.

At the pyro shoots I go to, we just have sheets of 3/4" plywood with bracing around the outsides, and some legs to angle them a bit. They'd work for probably all but a direct large whistle rocket strike. There are a few with plexiglass or polycarbonate built in to view through. Those get heavy quickly though. Something with a window may keep the prices down while maintaining functionality over using full sheet polycarbonate.

Adding more fuels should cool it off while still retaining respectable thrust. Your formula (in broken down components) is as follows:

KNO3 - 52.5
Charcoal - 20.5
FeTi - 20
S - 7

Swapping a few parts KNO3 and Charcoal should cool it down a bit. The swapping as opposed to just adding extra charcoal will maintain the 10 or 100 parts that some people are particularly fond of.

The other way to slow it down would be to use smaller drivers or a larger wheel. The tapered rammers is a great idea as well. If you feel like there is slag building up inside, some added milled meal can help to clear out the chamber, though a lot of it will increase the thrus

I included two drivers I happen to like below. The first is from Degn, and the second I really have no idea. I may have made it up, but I doubt it. It's pretty much a rocket composition with added metal. The ratios of airfloat and coarse charcoal and metal can be played with to balance thrust and get the effect you want. The less airfloat, the stronger the thrust will be.

Degn:
KNO3 - 18 (45%)
Steel - 12 (30%)
Sulfur - 5 (12.5%)
Airfloat C - 3 (7.5%)
mixed mesh C - 2 (5%)

The other one:
Meal - 70
Titanium - 10
Airfloat C - 10
Coarse C - 10