What is the ruling on omitting the first Tashahhud when making up many missed prayers, in order to lighten the burden of making them up, knowing that it is a Sunnah and can be compensated for by the prostration of forgetfulness (sujud al-sahw), which I do not perform? And do I have to repeat these prayers?

The first tashahhud and the prostration of forgetfulness (sujud al-sahw) must be observed when making up missed prayers. The first tashahhud is obligatory according to some scholars. Intentionally abandoning sunnahs (recommended acts) invalidates the prayer according to a significant number of scholars, while others consider it haram (forbidden) and require seeking forgiveness and repentance. If the first tashahhud is intentionally omitted, the correct view is to perform the prostration of forgetfulness. Omitting the prostration of forgetfulness is another deficiency in the prayer. Therefore, missed prayers must be made up in the most complete manner possible.