Choosing the right RDS instance type

To determine the appropriate memory size for your RDS instance, you will need to consider the size and usage patterns of your databases. From the information you provided, it seems that the majority of your databases are relatively small and have low usage, with the exception of Database 5, which is larger and has a higher usage rate.

Based on the information you provided, I would recommend starting with an RDS instance with at least 4GB of memory. This should provide enough memory to comfortably handle your smaller databases, while also providing some headroom for growth. However, to handle the load of Database 5, you may want to consider upgrading to a larger instance type or using a separate RDS instance solely for that database.

When deciding whether to use a separate RDS instance for Database 5, you will need to consider factors such as the amount of traffic it receives, the complexity of its queries, and the impact it could have on the performance of your other databases. If it is critical to your business and receives a lot of traffic, it may be best to isolate it in a separate instance to ensure optimal performance. However, if it is not heavily used or is not likely to impact the performance of your other databases, it may be fine to keep it in the same instance and simply increase the memory allocation.

Always try to have the Database of latest generation. For you, If the load is normal, then t4g.medium or db.m6g.large should work well. (As I am considering a normal load pattern). You might want to do the performance testing and then take a final call on the DB Instance size.