Documentation and and SQL scripts for downgrading Adobe Commerce to Magento Open Source
Please see the related blog post here for more information on this downgrade.
The key is that what's provided here is 'as-is'. It is the actual downgrade process used on an Adobe Commerce store which is still now running as Open Source without issue.
There is a very high chance that you will need to customise the processes and SQL scripts found here for it to be complete solution for your install, considering the Commerce functionality, and first and third party extensions you use.
At the very least test, test, test away from production before you attempt to downgrade a production system.
If you want to have the downgrade done for you, you can drop me a message using out contact form at https://developerconnection.co.uk/contact and we can start that conversation. You WILL need a competent developer to do this downgrade for you. Downgrading from Adobe Commerce to Magento Open Source is far from straight forward.