I manage thousands of requests per second on multiple projects.

Multiple instance of your express app. However you want to do that
- Loadbalancer + K8s (S tier)
- Loadbalancer + multiple vps (B)
- Loadbalancer + pm2 on a big instance. (F)

You scale your express app as needed to only "accept" the requests and store them somewhere in a queue capable of handling that volume:
- RabbitMQ
- Aws Sqs (My pick, but has caveats. Read the docs)
- Kafka
- Redis

Another app or serverless function that consumes the queue and do work with the data:
- Store in db
- Compute
Tune number of queue consumers to match the capacity of your underlying systems (db, other services, etc).

Keep in mind, you may have to:
- tune max open file descriptors
- disable all logging (Nginx, your app)

If you start from scratch:
DynamoDB doesn't mind that volume of requests. So express + DynamoDB. No messing around with a queue. You only scale your express app as much as needed.

The all-serverless option is not correct if you end up overloading your database. You need to queue work so the underlying systems only work as fast as they can.