Drop in sales after migration magento to shopify

My website was migrated from Magento to Shopify about 8 months ago and it has nearly killed my business. The drop in sales has been devastating. The developer assured me everything was OK, but clearly something has gone very wrong. Google Search Console and Google Analytics were only set up after the migration, so I don't have the information from before the move. I need somebody who can find historical data from other sources and work out what happened to my Google rankings and overall perform…