You can have SA permissions to SQL Server on RDS Custom.
RDS custom gives the maximum permission - which is a Sysadmin (SA) permission within the database and also an Administrative access to the Operating System. (Both are not possible in the standard RDS instance.)
When creating RDS Custom SQL Server in console, getting: Create DbInstance currently doesn't support parameter no-storage-encrypted for an RDS Custom SQLServer instance
Is it possible to use AWS RDS SQL Server as an AAG target from on premise primary?asked a year ago
RDS SQL Server - Using MSDN licensesAccepted Answerasked 5 years ago
Can you extend an availability group from on premises or EC2 to RDS Custom for SQL Server or use RDS Custom as a read replica?
In RDS Custom SQL Server, can I adjust the main database storage using custom EBS or similar?Accepted Answer
RDS Sql Server support for 15.0.4198.2asked 7 months ago
Sizing RDS SQL Server for existing SQL Server running on Windows ServerAccepted Answerasked 6 days ago
Can I change the service owner in RDS Custom for SQL Server?
In RDS Custom for SQL, can users or admins assume roles such as sysadmin?Accepted Answer
Amazon RDS Custom SQL-Server: Agent not runningAccepted Answerasked 2 months ago