Reducing Memory Pressure When Performing Parallel Staging Table Loads
If you take a look at my answer over here, I mention a new(ish) query hint called MAX_GRANT_PERCENT that you can use to further control memory grants at the query level. 

Documentation is here.

Assuming queries are in the default pool and you haven't altered it, right now they can ask for 25% of your 768 GB of RAM. That's a lot of memory.

You'll have to play with reducing the percentage until you find a balance between concurrency and not spilling to disk-ency.

Hope this helps!