I think you are underestimating how run down the East German economy was at the end, and how attractive West Germany was for East Germans. The problem was not simply down to debts, but the rotting infrastructure and antiquated production processes. The Politburo was warned in the middle 1980s that unless there were heavy investments into the crumbling ancient parts of the cities, they would literally rot away until the early 1990s.
the unforgettable Margaret Thatcher
I have quite a lot of respect for Thatcher, but it is obvious that she really dropped the ball during the negotiations for the German reunification. She was completely irrationial, unlike her usual calculating self. No wonder she was ousted shortly afterwards by her own party.
Is it true that one German chancellor was prepared to exchange West Berlin for certain western areas of Thüringen
That sounds very unlikely. Besides West Berlin was not under the authority of the FRG, but the occupying powers, so legally speaking it would have been impossible for him.
to get debt relief on the huge amounts of money that they owed,
The debts were not really the problem, because even though the GDR could no longer pay them back by 1989, they were relatively low. The GDR needed investments instead, and who would have invested into a country that just failed to uphold their debts?
