Multiple Transit Gateway Route Tables and Direct Connect Gateway Attachment

Clarifying a few terms so that it aligns with our documentation:

• Attachment: This is where you connect something (VPC, Direct Connect Gateway, VPN, etc.) to Transit Gateway.
• Association: Each attachment can have a single route table associated with it.
• Propagation: This is how a route table "learns" routes. Propagation can be automatic or you can manually assign static routes to route tables.

While you can have a route table for each VPC I would question whether that is a good idea or not - it can be a lot of extra work. If you were trying to set up an environment where the VPCs cannot communicate with each other but can communicate with your on premises network via Direct Connect then you can do that using only two route tables - one for the VPCs and one for the Direct Connect Gateway attachment.

However, to answer your question specifically: You don't need to attach Direct Connect Gateway to multiple route tables; you want to propagate the routes for your on premises network (which are reachable via Direct Connect and Direct Connect Gateway) to all the route tables which are associated with the VPC attachments. You can absolutely do that.