SQL Server High CPU usage and RESOURCE_SEMAPHORE waits
What are your thoughts and experience on above ?


Why would a statement inside SP that usually completes within 10 ms for months, starts to experience RESOURCE_SEMAPHORE wait and complete in 4000-8000 ms ?

CPU pressure caused by bad plans.  You should track and manage plan stability with Query Store, as well as investigating bad plans and remediating with additional index and statistics, and perhaps changes to the queries.

Can constant RESOURCE_SEMAPHORE waits and Memory Grants Pending cause pressure on CPU just to allocate workspace memory for queries?

No, it's the other way around.  Bad plans are resource intensive, driving both large memory allocations and CPU use.