It's possible the impression share loss due to rank includes losing out on impressions that the campaign is eligible for but are very broad so you would not want to show for them anyway.
Why Google Ads is showing a reporting a different conversion value from Shopify.
If you have extra ad spend to work with...
This largely depends on ad spend. Here’s what we mean:
Conversion Actions - To be able to track a conversion, we would need a "trigger" for that conversion action.
Sometimes when the budget is limited, having too many products in the shopping ads means the algorithm has trouble gathering conversion patterns in order to optimize because each conversion on each shopping ad/product is too spread out across the weeks/months, so the answer is to temporarily exclude products until the algorithm can get the hang of things. Then, with scale later on, those products' shopping ads can be re-included. Just because we exclude a product, doesn't mean it won't be purchased. It just means it might not work well as a shopping ad & the money could be better spent on other shopping ads to bring people to the site...but those people still might buy these products.
Shared budgets are not ideal but they can be useful.