All configuration you have shared are correct.
After set backorder No then it will never go to out of stock status.
If you are set backorder and product is already out of stock then it will not work, so you need to change product out of stock as well and save products again.
Also run below command after set all configuration:
php bin/magento indexer:reindex
php bin/magento cache:flush
It may help you to resolve issue.
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