sometimes heating the surrounding area will loosen something.

i myself most of the time will cut with a saw because i'm always afraid i'm going to crack it.

brass cuts and drills easy.

Normally I try a larger wrench if that doesn't work then I use a chisel cave in the threads remove the plug snake the line and replace with a lead fits all plug.

Now for the disclaimer.

NEVER ,EVER use an open flame to loosen any sewer cap/ plug as the sewer fumes can contain highly flammable vapors such as methane and hydrogen sulfide.

You can try penetrating oils like liquid wrench (Seldom works) or WD 40 Or a Wrench followed by hitting the handle carefully with a lump hammer or

a persuader a pipe placed on the wrench handle to give more leverage.

One other trick of the trade it try to tighten the plug then try backing off.

again keep open flames away from sanitary lines unless very well ventilated as gas pockets are highly dangerous

Will try this weekend - thanks

Any ideas of what the brown hardened material is?

Old joint compound