I agree with SQLGuru. You should time-box your tuning efforts.
If you have not fixed the problem by then, getting a SQL expert could have the problem fixed in a day and give valuable guidence on how to troubleshoot future problems.
If you are worried about costs, you could raise a support call with MS. This costs about GBP £300 if it is not included with your license agreement. My experience of PSS is they are very good at identifying the cause of the problem.
Original author: https://github.com/SQL-FineBuild/Common/wiki/ 1-click install and best practice configuration of SQL Server 2019, 2017 2016, 2014, 2012, 2008 R2, 2008 and 2005.
When I give food to the poor they call me a saint. When I ask why they are poor they call me a communist - Archbishop Hélder Câmara