Zend certified PHP/Magento developer

We lost 75% of sales after switching from Magento 1 to Magento 2. How to get back up to 100%?

Magento 2.4.2-p1
Smartwave Porto theme
Extensions from Amasty

We have migrated from Magento 1.9.4 to Magento 2.4.2-p1 using the Magento Migration Tool about a week ago but now we are getting 75% less sales on both of the websites/stores.

I randomly checked the URLs of 100 products and out of those, 30 resolved correctly on the new Magento 2 website and 70 went to a 404 Error on the new Magento website. I will set up permanent redirects for these.

What else can we do to get back to the same amount of customer visits/sales that we had before the Migration? Would switching back to Magento 1 bring the sales back?