2.21What your previous QA work was actually testing

This is worth recognizing because you are closer to development than it might feel.

When you tested:

MuleSoft
   ↓
Salesforce

you were probably indirectly validating things like:

HTTP request contract
        ↓
Mule routing
        ↓
DataWeave mappings
        ↓
Salesforce Connector configuration
        ↓
SOQL / object model
        ↓
field mapping
        ↓
upsert semantics
        ↓
Salesforce response

As a developer, you're simply moving one layer inside the box.

Instead of:

Send request
↓
verify Salesforce

you're implementing:

Receive request
↓
transform
↓
call Salesforce
↓
handle result

That transition is quite manageable.