Traffic drop by 49.75% after migration from Magento to Shopify?

We hired a team and designed the theme for on Shopify which had been previously on Magento. On Sept. 8th we finished the theme, created all 301 redirects (products, categories to collections, pages, WordPress blog posts to Shopify blog posts), and migrated everything over to Shopify and since then we’ve seen a constant drop in traffic as compared to when the site was on Magento. (Sep 9, 2020-Nov 19, 2020 comparison against previous period) (Overal traffic drop by 49.75%) (Straight traffic drop ever since Sep 9, 2020)

As you can see from the screenshots of Google Analytics, there’s a significant sharp drop in traffic immediately after we migrated. We have been trying to find the culprit here but as it looks like an overall drop across ALL traffic acquisition channels, therefore, we don’t have any clue as to why this happens?

There’s not only the drop in traffic but also a drop in transactions & revenues on the same scale. It’s as frustrating as it is weird because half of external traffic sources seem to suddenly cease arriving on my site as if they know my site is different so they suddenly refuse to land on our site?

We still have the old site online if you want to compare:

At first we thought it’s just a coincidence of traffic fluctuations and it would return to normal after a few days or weeks but it seems we are wrong.

Also as a comparison, here’s the stats of May 19, 2019-Nov 19, 2019 in GA for when it’s still on Magento, so we can rule out the season factor. Traffic is actually climbing as we sell through summer to fall. (May 19, 2019-Nov 19, 2019 no traffic drop at all)

We are truly desperate at this point with so much invested in marketing and re-design but results cut in half. Any help would be tremendously appreciated! Thank you!