Optimizing TGW Routing for Select VPC Subnets in DX Communication Scenario

A client has a specific requirement to establish communication between a VPC and an on-premises network via Direct Connect (DX), utilizing a Transit Gateway (TGW). This communication is necessary for a subset of private subnets within the VPC; specifically, out of the existing 6 private subnets, only 3 need to establish connectivity to the on-premises environment.

Here's the sequence of actions I've undertaken to address this requirement:

1. I initiated the creation of a TGW attachment dedicated to the target VPC.

2. I crafted and associated a route table with the above attachment to facilitate the connection.

My current deliberation centers around the need to set up propagation within the TGW route table. My understanding is that the entire CIDR range of the VPC would be disseminated. Nevertheless, I'm contemplating if there's a method to permit solely the 3 specific subnets to engage with the on-premises infrastructure. Furthermore, I'm exploring the feasibility of condensing the route information for these subnets, streamlining the connection to the on-premises network.

I'm seeking guidance on how to effectively address this scenario, ensuring that only the designated 3 subnets are authorized for communication while concurrently optimizing the route configuration.