This is right in general, but it is good to compare the peak area for both the weight loss in (DSC plots). That will give you the number of molecules decomposed at these temperatures for better explanation.

Decomposition commences around 600 degrees, so the departure of carboxylaye leaving aromatic ring intact behind seems unlikely. Secondly, there are more than two steps (See derivative), probably it follows departure of aromatic ring then carboxylate, leaving the Li2CO3 behind. One more thing may be done: record TGA (with separate samples) up to 656, 700 and 823 deg. and try to analyze the remains by elemental analysis.

Thank you!!!!