As I had mentioned earlier you cannot do that without using an Update With Aggregation Pipeline. The operators you can use within the pipeline are Aggregation Pipeline Operators - not the Update Operators ($inc, $push and $set).
You can use the Aggregation Pipeline Operators - $add instead of $inc, $concatArrays instead of $push and then assign the totalQuantity to the currentQuantity.
You need to get familiar with using the Aggregation queries to understand and work with Updates With Aggregation Pipeline. Here are some example posts: