I Wish I Would Have Known #3: Quick Create Forms

Although you can define multiple quick create forms, only one quick create form can be used by everyone. The form everyone will use is set using the form order. Quick create forms cannot be assigned to security roles and they do not provide the capability for the user to switch forms.

https://docs.microsoft.com/en-us/powerapps/maker/model-driven-apps/create-edit-quick-create-forms#create-a-quick-create-form

I Wish I Would Have Known #1: Fetchxml VS Query Expression

It totally depends on your plugin style: In case for early-bound, I like using LINQ because it’s strongly typed and It helps development speed. Certain benchmark test have proved that Fetchxml is slightly faster than QueryExpression.

As per Microsoft official docs, there is no visible performance difference but just a personal preference of the developer.