As for fat gain not sure, rebound from my understanding is after stopping T3 for two weeks it may come back, but not to where it was prior. So just like with Testosterone you may be within range in a certain amount of time at 300 TT but when one was 800TT before the cycle he is still gonna suffer.
Thyroid comes back fine in almost all cases and from what you posted here, you're using rebound in the wrong context bc you don't know what the word means in a medical context.