I have been using composite API on various integrations but recently arrived at couple of use cases where conditional execution of a subRequests or population of values on child/related records.  Following are my use cases

• Skip Fields population or use default value on reference record when Get API not found specified record
• Skip Child records being created/updated based on the reference-record field values
• Conditional field population ex. if Account does not exist create case with Anonymous Accounts with different reference record

Have been reading couple of articles but did not get any alternatives. Posting to see if anyone achieved these scenarios with Standard Composite API.