I couldn't find any way to bulk edit sales channels after uploading 10,000 additional products.
CSV product upload doesn't support sales channel updates.
The best solution I found after contacting shopify was to use the product bulk editor, and click the first item in the "available on xxxx" column, and using shift click on the last item then pressing space bar a couple of times to select all the rows.
This isn't perfect, the All Products listing page doesn't have paging as numbers, just left and right. So you will need to use the back button on the browser to get back to the list of products you're up to editing, then again click the next arrow key (K).
This isn't ideal for a shop with a lot of products - I would personally recommend creating your own XML feed based on a collection with the right filtering and a theme template to deliver the collection as an XML feed. This can be used for both Facebook feeds, and also the Google Merchant.
I would welcome if anyone is more familiar with this and knows a way to update on sales channels quickly.
The free app you are using is not advanced and recommend you use a paid version if you want advanced editing.
Alternatively, use GMC feed rules: https://support.google.com/merchants/answer/7450276?hl=en
Or use supplement feeds: https://support.google.com/merchants/answer/7439058?hl=en