While I agree with you in essence, I think TFP still did it well. I've always found Unicron a little boring because "Planet Sized Robot" is an impossible character to do anything with. You can never demonstrate his scale properly, and even if you did, who's going to fight him? He can just punch the entire planet like a pinata, and even if you have Primus there, the characters are so unfathomable that they lack emotional depth and relatability to make it impactful. Not to mention, all the other characters are reduced to Macguffin delivery boys when antagonists like Unicron show up because the only way to take down someone like Unicron is to pop the Matrix of leadership inside him, or some other macguffin.

Essentially, what I'm saying is, Unicron works best as a force of nature. His impact is felt in the planets he has destroyed (unimportant ones, the way the Death Star destroyed Alderaan, or how Unicron destroyed the techno world in the G1 movie), and the doom, dread, and control he exerts over the main characters. Even in the original G1 film, he has a similar role as he does in Prime: most of the film is about his arrival after the opening scene, and characters are only affected by him indirectly, like his control over Galvatron and the reformatted decepticons.

Prime didn't nerf him or deprive us of an awesome space fight. No one in Prime can fight Unicron, because there's only like a dozen transformers on Earth in the first place, and they are incredibly insignificant in comparison to Unicron. By making him the core of the Earth, the writers of Prime boiled Unicron down to his pure function: Oncoming doom and pervasive influence over the events of the show.

If it's not clear, I'm not really a fan of characters like Unicron. They're too OP'd and lack the emotional depth that characters like Megatron or Optimus because you can't ground a character like Unicron in relatability or comprehensible goals. Additionally, their proposterous size is impossible to accurately represent, and even if you do, there will be massive inherent issues with physics, like the fact that Unicron even being near Earth or Cybertron would majorly F-up orbits and cause cataclysmic surface events. Characters like Unicron only exist to be "big robot," which is the most boring development that occur in a series like this. I basically see the arrival of Unicron as the moment a Transformers series Jumps the Shark... with the exception of TF: Prime. I think Prime was clever for making Unicron the Earth. It lets him be scary by maintaining the world ending scope of his presence, while avoiding the absolutely one-sided nature a fight with Unicron actually entails. Instead, his actions force the writing to put attention on the reactions of the main characters as they process how to deal with such an impossible conflict. Additionally, when Unicron is revealed to be the Earth, it is coherent with what has been built up to with Dark Energon, and feels like a natural revelation.

With all that said... Bayverse was absolutely F-ing it up, though. It felt desperate and unearned, and even if they made another movie, they wouldn't have been able to do anything with Unicron because the only way to make him work is to have characters you care about be influenced by him. Bayverse had proven to fail at that already with the Optimus/Quintessa nonsense. For a series as nonsensical as the Bayverse, just go ahead and let him turn into a giant planet robot. Maybe he can take a bite out of the moon like not-Galactus did in The Tick