could have perfectly ringed out Waka whenever he wanted

Not really, Fei made like 3 different ring out attempts and failed all of them.

but Fei died due to his own negligence trought the whole match and abusing of DD, not really because Waka was a formidable opponent or anything like that.

Fei died cause Waka managed to outlast him and forced him to use DD as the one time Fei dropped it Waka landed a karate dash followed by a blastcore that grazed Fei, it wasnt stupidity of Fei using DD he was legit scared Waka would get him with a blastcore

Fei was screwing around through basically 95% of the fight

Fei received a lot of damage from Waka even when he was not fucking around, he tried to ring out Waka with a flashing steel which Waka almost reverses and throws Fei out of the ring, that led to Waka grappling and kneeing Fei. Waka also surprised Fei with a karate dash which led to Fei taking more damage and led to him almost getting blastcored and having to barely avoid it thanks to DD reaction speed and dancing snake. And at the end he was forced to use max output to get Waka off of him which yes happened for letting Waka grab him, but still Waka almost beats Fei multiple times. Lastly you had the final blastcore that scratched Feis face and would have gotten him had not been for DD reaction speed. To me it was definitely high diff.