Looking for some help/guidance/suggestions.

Hey - from what you mentioned it sounds like when they upgraded the site they botched the migration. Interested to know if the URLs changed during the "update"? If they did and weren't redirected properly, that would have had major impact on traffic, and also revenue. If this is the case, there could be some quick wins by implementing some 301s from old broken pages to new ones.

It's amazing how many companies destroy their business to save a few bucks per month or because they want "complete control" of the look or functionality of a site (that complete control comes with a lot of complete responsibility many companies aren't equipped to deal with).

It sure sounds to me like the first step would be moving the slow, clunky site off of WordPress and back to what they were using before or any other modern platform actually built for eCommerce (Shopify, BigCommerce, 3DCart, etc.).

Yes clean up the web site clunk first, even if it is still WP, because finding and redirecting old links into the new places on the site that is clunky will just make it worse.

As far as redirects and their age...if too much time has passed where bad links have existed on very valid and qualified search refs or back links to non-existent info or pages at your site, they may already be removed, so redirecting becomes pointless. If there are any stray links left, get those redirected right away, once you know the site is stable enough.

Going back to the way it was at 3K a day with the same platform is likely not possible, but focusing on what was creating the 3K and moving forward with that seems quite logical. Since the SEO people and their changes have taken you the other way.

It is possible to completely migrate an e-commerce store to the same type of e-commerce store using another url and using another platform other than WP. This is done when an old store must be left in place and not used anymore, but you need to be very aware of what you are doing through every step because you have an active store that you are moving.

Keep the old site and just optimize it properly man, no need to start from scratch. On-page SEO goes a long way. Thousands of ways to increase WP sites' loading speeds (which will decrease bounce rates by a ton in most cases)