Can I use Azure Data Lake in place of Data Warehouse to separate Compute and Storage of RDBMS analytics
Your approach sounds reasonable. Many of the analytics-oriented DBMS allow access to data lake files in situ. You're just reproducing a similar feature in user-space.
IIRC ADF's data flow uses a DataBricks cluster under the covers. This may be a simpler implementation than writing dataframes in a notebook and getting it to run in 'Bricks.
I think most opt for a load then process approach because SQL makes such manipulation easy and Synapse is optimised to perform it.
If your source database is SQL Server you may want to consider Change Data Capture for watermarks / delta tracking.