Can it be said that missing the Asr prayer is the biggest sin after shirk/kufr, because missing Asr nullifies all the good deeds. (that is if we said Asr isnt shirk/kufr)

No. The hadith is about deliberately abandoning it, and Imam Ibn Baz said the ruling is one for all five prayers, ʿAsr being singled out only for the severity of the warning.