What is up with db.m series?

The db.m5.large instance type is indeed available for Amazon RDS, despite the difficulty you're experiencing in finding it. This instance type is part of the general purpose DB instance class family, which is designed for a wide range of database workloads.

The db.m5.large instance is specifically mentioned as a supported instance type for Amazon RDS on AWS Outposts. It's also included in the range of supported instance types for RDS Custom for SQL Server in the Standard Edition category (db.m5.large–db.m5.24xlarge).

If you're unable to see this instance type when trying to launch an instance or in the calculator, there could be a few reasons for this:

1. Regional availability: Not all instance types are available in all AWS regions. You may want to check if it's available in your specific region.

2. Service-specific offerings: While db.m5.large is available for RDS, its availability might differ between RDS and EC2 services.

3. Recent changes or temporary issues: AWS occasionally updates its offerings, which could affect visibility temporarily.

4. Console or tool-specific limitations: Sometimes, certain tools or interfaces might not show all available options.

For your reservation estimate, you should be able to include the db.m5.large instance type, as it is a valid and supported option for RDS. If you continue to have issues locating this instance type, it would be advisable to contact AWS support for clarification, as they can provide the most up-to-date and region-specific information.

Remember that instance type availability can change, and it's always a good idea to double-check the current offerings when planning for reservations or new deployments.
Sources
Supported DB instance classes for Amazon RDS on AWS Outposts - Amazon Relational Database Service
DB instance class support for RDS Custom for SQL Server - Amazon Relational Database Service