4.32What would you automate as a tester?

This is where you can connect your existing expertise directly.

For an Upsert integration I'd want tests for:

new external ID
→ Salesforce record created

existing external ID
→ same record updated

same request repeated
→ no duplicate

optional field missing
→ existing Salesforce field preserved

explicit clearing operation
→ field cleared if contract permits it

invalid required field
→ useful error

Salesforce unavailable
→ retry/error behavior

mixed-validity collection
→ partial failures correctly reported

Account + Contacts
→ relationships correct

Those tests are basically the developer acceptance criteria for your connector design.