Solar flare and corona stuff is like large planet level, Emperor's strongest attack when he killed Horus was supernova level IIRC.
No, it was described "being like" a supernova. And this happened inside of the Vengeful Spirit which was hovering over the Imperial Palace, on Terra. Brakiss' solar flare feat is far more impressive because we actually see him do this. He's an ant to any named Force-User of worth; Naga Sadow had done the same to multiple stars and could with the Sith Corsair, cause multiple supernovas in multiple star systems with its magic amplifying his power.
Sidious powering up after killing Plagueis caused a disturbance and quake in the Force that shook stars.
He could tackle with C'tan who eat stars and spawn black holes.
C'tan have routinely been taken down by Necron Overlords like Trayzan the Infinite. Their Shards are quite weak and it was directly shown to us that the Emperor only fought a weakened shard of an already weakened Void Dragon after he had been blasted by a dozen or more Blackstone Fortresses. The C'tan have never manifested the power people have claimed they have. Then again VSB has claimed "solar system" level Sigismund and galaxy level Ahriman, so I guess that's where its coming from.
When it comes down to the final stage between Force Ghost Palps vs Warp Child Emps, well Emps has fought the universal embodiments of all emotions for ten thousand years and held them back from consuming the galaxy and potentially universe.
The Chaos Gods are not universal, not even close.
The Shadow of the Warp created by the Tyranids who have already devoured other galaxies before coming to the Milky Way Galaxy in the late 41st century are literally something new to both daemons and the Chaos Gods themselves.
The Chaos Gods are entirely bound within the Warp/Immaterium mirroring the Milky Way Galaxy.
Emprah also has a massive skill and experience advantage, IIRC masterminding tens of millennia worth of martial, tactical, combative, and scientific knowledge. Even the composite of Palpatine's knowledge only goes back ~3500 years.
Palpatine literally had mastered every single Jedi, Dark Jedi, and Sith ability created over 35,000+ years. He also was so good at this he was inventing powers and abilities purely for his own sport as per the Dark Empire handbook:
Palpatine has spent decades studying the most arcane and esoteric Jedi disciplines. It is believed that he has mastered nearly all the known powers, previously unknown powers, and devises new ones at his pleasure.
So here you have it:
- Palpatine kills his master which causes a planet as massive as Coruscant to shake and cause near and distant stars to pulsate from his power up
- maintains the shroud of the dark side by himself even after Maul is seemingly lost and Plagueis dead against the entire light side of the Force manifested by the Jedi Order for over 13+ years and disabling their long-range clairvoyance and precoginition across the galaxy
- muddles the future so not even the most powerful Jedi can scry it
- his reborn Clone required every single Jedi in history along with the combined power of the entire light side of the Force to drag his spirit to the Netherworld of the Force
- can casually brainwash, mind-control, and telepathically alter trillions of minds without trying as he did so just to hide his personal Super Star Destroyer within the cityscape of Coruscant
- even a trained and powerful Sith Lord like Plagueis upon encountering Palpatine before he had any training in the dark side or as a Sith apprentice had already massively reflexively built psychic barriers that prevented Plagueis from probing his mind; mind you Plagueis is so powerful that his mere presence on Naboo created the worst snowstorm and winter in its entire recorded history