All sources I found which directly state or heavily imply that Mace Windu legitimately defeated Darth Sidious
Compilation of Disney released sources which directly state/imply that Mace Windu defeated Sidious legitimately:
>Windu's final mission was an ill-fated attempt to arrest Chancellor Palpatine which would have been successful had it not been for the last minute intervention by Anakin Skywalker
>When the Jedi discover his true identity, Master Mace Windu confronts the Sith Lord. Sidious would have perished if not for the intervention of Anakin Skywalker
-Star Wars: Card Trader (2015)
>In the final days of the war, he confronted and defeated Chancellor Palpatine (in actuality Darth Sidious) in a difficult duel, only to be betrayed by Anakin Skywalker
-Topps Star Wars Evolution (2016)
>Mace Windu was able to hold his own against Palpatine, but the Sith Lord quickly defeated the other Jedi. Disobeying orders, Anakin arrived just in time to see Mace Windu about to overcome Darth Sidious
- Revenge of the Sith Read-Along Storybook and CD (2017)
>Windu nearly overpowered Darth Sidious, forcing him into a corner and holding him at blade point
- Star Wars Lightsabers: A Guide to Weapons of the Force (2018)
>Mace Windu was also the only one to defeat Darth Sidious in a duel
>However, the toughest confrontation was between Windu and Palpatine. At one point Windu turned the duel in his favour and demonstrated why he deserved the title of "Champion of the Jedi Order"
- Star Wars: Geektionary: The Galaxy from A - Z (2018)
>But his action results in a fight. Windu prevails. But Anakin appears on the scene and turns the situation around, allowing Palpatine to win
- Topps Star Wars Galactic Files (2018)
>Mace forms a Jedi strike team to arrest him. While his comrades fall, Mace is nearly successful. However, Anakin arrives and defends Palpatine, allowing an opening for Palpatine to kill Mace
- Star Wars: The Complete Visual Dictionary, New Edition (2018)
>Mace Windu battled Supreme Chancellor Palpatine after the latter was revealed to be the mysterious Sith Lord, Darth Sidious. While Windu took the upper-hand, an interruption from Anakin Skywalker turned the tide and allowed Darth Sidious to destroy the revered Jedi Master
- Topps Star Wars Galactic Moments Countdown to Episode 9 (2018)
>Master Windu led a group of Jedi Masters to arrest the corrupt leader, and had my father not betrayed him, he might have bested the Dark Lord in Combat
Star Wars: The Secrets of the Jedi (2019)
>By the time Anakin arrived on the scene, three of the four Jedi Masters lay dead, cut down by Palpatine's crimson lightsaber blade before it was lost. The Sith Lord's fate was now in Mace Windu's hands alone
- Skywalker: A Family at War (2021)
>Windu disarms Palpatine, and forces the Sith Lord into the corner of his spacious office window
-Star Wars Databank
There are a lot more but I wasn't able to find their origin or year or release so I omitted them from this compilation.
Next part is about the force lightning exchange. Vast majority of sources state that Darth Sidious got disfigured by his own force lightning which heavily implies that he was going all out and contradict any "toying" statements. We even have a source which directly states this:
>Palpatine was using force lightning to try to defeat Master Windu, but it was rapidly draining him of all his energy
-The Prequel Trilogy Stories (2017)
Below are all the sources I found which state that Sidious got disfigured by his own Force Lightning:
>Palpatine was badly scarred in the fight
- Revenge of the Sith Read-Along Storybook and CD (2017)
>Mace Windu succeeded in disarming him after a savage duel, and used his lightsaber to deflect the Force lightning Palpatine struck at him. The rebound blast disfigured Palpatine
- Star Wars: Geektionary: The Galaxy from A - Z (2018)
>Windu deflects the lightning back towards Palpatine with his lightsaber, causing Palpatine's face to become scarred
- Star Wars: A Galaxy at War (2018)
>Darth Sidious was permanently scarred by his own Force lightning when Mace Windu tried to deflect the Sith Lord's attack
- Star Wars The Secrets of the Jedi (2019)
>A Jedi task force learns that Palpatine is a Sith Lord and tries to capture him. Palpatine kills them but his own Force lightning leaves him disfigured
- Ultimate Star Wars, New Edition (2019)
>Dark Lightning emanated from Palpatine's fingerprints, ricocheting off Windu's blade to leave Darth Sidious deformed
- Skywalker: A Family at War (2021)
>Face scarred by his own Force lightning during a duel with Jedi Mace Windu
- Star Wars The Dark Side Pocket Expert (2023)
>Mace cornered Sidious, and the Sith Lord unleashed torrents of dark Force lightning at his opponent, some of which were deflected back at Sidious, forever scarring his face
- Star Wars Databank
There are more stating the same but once again I wasn't able to find their origin and year of release hence why I omitted them from here. Thats all for now