It is a highly recommended Sunnah to pray after giving Iqama. If you don’t, your prayer is accepted. As for the first Takbeer, this is a pillar and if one misses it or forgets it, his prayer is invalid. As for Sami’a Allah liman Hamidah, this is mandatory to say. As for the Athan if praying individually, it is recommended but not mandatory.
And Allah knows best