What must I do for my Umrah to be accepted, knowing that I performed the two rak'ahs of Tawaf after the Sa'i, and forgot to shave, then I exited ihram before doing so, then remembered and shaved, and did not commit any of the prohibitions of ihram after the first tahallul?

Your 'Umrah is valid, Allah willing, because the two rak'ahs of Tawaf are Sunnah according to the جمهور of scholars. As for exiting the state of ihram, it is not done by bathing and changing clothes, but rather by shaving or cutting the hair with the intention of exiting ihram. Furthermore, an ignorant person who commits prohibitions of ihram is excused according to some scholars.