Stand the barrel overnight in a cup of Kroil. Probably the safest and cheapest fix.
I heard a trick of packing wadding in barrel just below the choke tube and then dropping dry ice inside. If you warm the barrel a little bit first, the tube will slightly shrink. This is also how machine shops fit tight bearings.
I had one stick in the field and I sacrificed the tube. It did teach my son a valuable lesson about cleaning the threads.
I would do a kroil and heat gun combination. It will come out, probably not as stuck as it seems. I've been using a loctite silver anti-seize #ss-13850 for years on my tc threads and never had issues. Have gotten alot of them out for guys without causing any damage. Soak the choke tube end in a cup of kroil over night. Diesel fuel/fuel oil in a cup won't hurt either.
I had a pilot jet stuck in a carburetor for a number of years. Every once in a while I'd pull it off the cycle and try to get it out. Somewhere along the way, I was introduced to Kroil and one day I decided I'd give it a try. You guessed it, it came out the next morning. As long as you have enough patience to let it work, I think you'll be happy. If I find something rusted, Kroil is the first thing I reach for. Good luck.
FYI - When I had a stuck tube in a Browning I soaked the end of the barrel in PB Blaster for about 24 hours. The choke tube did come out but it permanently discolored the end of my barrel. It wasn't really bad but you could certainly see the last 3 inches of my barrel was darker than the rest and there was a line around the barrel where the oil level was. It happened on an inexpensive BT99 so it wasn't the end of the world, but if it had been my Seitz I would have been pissed! I don't know if kroil would do the same or not, but I would cautious.
A friend of mine recently had a similar problem. He wrapped multiple layers of duct tabs around exposed portion of choke, soaked it in Kroil over night, placed barrel in a padded vice and applied torque with a wrench. Worked. I have 2 CG guns. I NEVER leave the chokes in the gun after a shoot. I learned the hard way after I was unable to remove them from my 28 gauge. The CG facility is near me. I went to them for help. They got them out alright,however, the chokes were ruined and needed to be replaced. CG recommended they be take out after a shoot. Something about what chokes are made of. Irv