Staging tables are fine but they don't buy you much if you're eventually having to load into the final table anyway. If your dataset is clean reliable or you have proper error handling in your loader (e.g. SSIS) then loading into the final table directly can far and away outperform loading into a staging table first and then using T-SQL to load into your final table.

MERGE might seem like a good choice conceptually but there is a ton of evidence out there that shows that INSERT followed by UPDATE inside a transaction is usually to be a better performer when handling large datasets. Test for yourself, of course. Also worth mentioning, there were more than its fair share of bugs opened against MERGE so make sure you have the latest patches and watch out for the ones that are still active as well.