Support the SQL standard MERGE statement. Essentially it's like INSERT with the CONFLICT clause, but also offers the possibility to do DELETEs/UPDATEs. Quite a few DBMS support this. The concurrency guarantees of the statement AFAIU allows such a statement to be implemented by executing the individual parts as separate statements, so emulation should be possible.

Also see:

• https://en.wikibooks.org/wiki/Structured_Query_Language/MERGE
• https://en.wikipedia.org/wiki/Merge_(SQL)

Syntax-wise I would suggest we follow the SQL standard approach which seems sane to me.

merge_statement : MERGE INTO entityName (AS identifier)? USING fromRoot ON predicate merge_when_clause+;
merge_when_clause
 : WHEN MATCHED (AND predicate)? THEN (UPDATE setClause | DELETE)
 | WHEN NOT MATCHED (AND predicate)? THEN insertStatement
 ;