I agree that writing relevant descriptions for such a large site can be a tedious and long process.
We also have an ecommerce joomla site that has over 2000 products listed. When we took over the site it had no SEF url's, almost all the meta tag descriptions and titles were the same.
The bounce rate when we started was approx. 60% and since we have started working through each product and changing SEF Links, Meta Title and Meta Description tags related to specific keyword search terms for each product our bounce rate has dropped to 25%. Thus we are getting much more relevant traffic.
So good luck with doing the changes as it will be worth while in the long term.