Getting another column from same row which has first non-null value in column

Question

I have a SQL table like this and I want to find the average adjusted amt for products partitioned by store_id that looks like this

Here, I need to compute the adj_amt which is the product of the previous two columns. For this, I need to fill the nulls in the avg_quantity with the first non_null value in the partition. The query I use is below.

select 
CASE WHEN av_quantity is null then
    # the boolen here is for non-null values
    first_value(av_quantity, True) over (partition by store_no order by product_id 
    range between current row and unbounded following
    )
else av_quantity
end as adj_av_quantity

I'm having trouble with the SQL required to get the adjusted cost, since its not pulling the first non_null value for factor but still fetches it based on the same row for the adj_av_quantity. any thoughts on how I could do this?

FYI I've simplified the data here. The actual dataset is pretty huge (> 125 million rows with 800+ columns) so I won't be able to use joins and have to do this via window functions. I'm using spark-sql