Please post the most common queries.

Also, why Datetimeoffset? It's not a common data type choice, especially since it's not daylight savings aware, which reduces the usefulness of storing a time zone

Why is UserID an NVarchar(Max)? I can't recall last time I saw a userID over 4000 characters long.

Why is StockTransactionType nvarchar(max)? You really have types that are several thousand characters long?

Why are there ID columns which are nvarchar?

Fixing what looks like some design problems will get you much better performance gains than partitioning. The MAX data types are probably killing your query performance, they and the nvarchar IDs may well be why the table is so hard to query.