Since the entire “Ultchtar” paragraph was made into Thundercracker, I had a little ‘what if’ idea pop into my mind of Thundercracker originally being the Decepticon second in command, and Starscream later replacing him with some name changes to cover it up because Thundercracker isn’t exactly the ‘ideal Decepticon’
Thundercrackers entire theme was based on meaning, superiority, and fear, that would explain him joining even earlier then Starscream if he just wanted direction his life, even though the Decepticons weren’t a huge power yet. It would also explain why Megatron gave him personal attention to make him feel superior and join if he was one of the first. Also I think it would add a nice, personal, reason on why his biggest fear is Megatron and how it prevents him from leaving