Hi,
We moved from Magento to Shopify. We need to make sure our GA-4, our merchant feed and our Adwords are all configured correctly to optimise for Google. Then test our basic brand ads to make sure they are working as appropriate.
Also, we would have a large number of NotFound pages and need to redirect them to the new URLs from Shopify.
Looking for help with this. We are a tiny business, so looking for some affordable help. 
Hello ShopFS, thank you for your question in the FeedArmy AMA Event.
When migrating to another eCommerce platform, expect poor performance, or at least lower performance for a while. Depending on how well you have done the migration, the poor performance can be just a few days, or even weeks or months.
So ensure if possible to match the product ID between the old feed and the new feed. This means don’t use the google and youtube app, because that is not possible. Instead use an app such as https://apps.shopify.com/multiple-google-shopping-feeds
Where you can change the product ID to the SKU, this means add the old product id, in the SKU field inside Shopify.
And make sure you match as much of the other attributes as possible.
Also ensure you have your conversion tracking setup, which you can follow here: https://www.youtube.com/watch?v=GflbZL2wyK8
If you have done these two things, you should be able to minimize your performance drop.
As to redirects, make sure that all shopify product landing page links, do not redirect. Google does not allow this, all product URLs submitted must be the final destination.