Is it valid to perform a comprehensive ghusl with the intention of repenting from everything that necessitates disbelief, whether due to ignorance or negligence, and is it obligatory to pronounce the two Shahadas before ghusl?

Congratulations on your repentance. Know that repentance from all sins in a general sense is sufficient, and you do not need to renew your repentance every time you recall something from your past polytheistic acts.

The ruling of kufr (disbelief) is not a light matter. A Muslim is not deemed a disbeliever unless they commit a statement, act, or belief that the Qur'an and Sunnah indicate to be major kufr, or upon which scholars have reached a consensus as major kufr, along with the fulfillment of the conditions for declaring kufr and the absence of its impediments, such as being an adult, sane, acting by choice, and not excused by ignorance or misinterpretation.

We advise you to distance yourself from waswasa (whispers/obsessive thoughts) and ignore them completely, because they are among the greatest dangers of Satan. Be eager to learn Islamic knowledge, study the correct creed, and seek righteous companionship. Increase in acts of obedience, remembrance of Allah, seeking forgiveness, and the reported supplication: "O Allah, I seek refuge in You from associating partners with You knowingly, and I seek Your forgiveness for what I do not know."

The repentance of an apostate is by their embracing Islam, so they pronounce the two testimonies of faith. If their apostasy was by denying an obligatory act, then their repentance involves affirming the denied act along with the two testimonies. Ghusl (ritual bath) is not a condition for the validity of repentance from disbelief; rather, it is obligatory to hasten to embrace Islam, and it is not permissible to delay it for ghusl.