The documentation is a little ambiguous:

"Row-level concurrency is only supported on tables without partitioning, which includes tables with liquid clustering."

https://docs.databricks.com/en/release-notes/runtime/14.2.html

 Tables with liquid clustering enabled support row-level concurrency in Databricks Runtime 13.3 LTS and above. Row-level concurrency is generally available in Databricks Runtime 14.2 and above for all tables with deletion vectors enabled.

https://docs.databricks.com/en/delta/clustering.html

Also, is there a method to enable cluster-on-write for MERGE INTO statements?

Most operations do not automatically cluster data on write. Operations that cluster on write include the following:

• INSERT INTO operations

• CTAS statements

• COPY INTO from Parquet format

• spark.write.format("delta").mode("append")