Architecture for transactional web platform

Good day.

I am making a configuration in which the firewall sends the requests to the loadbalancer and this in turn internally redirects the traffic to the backend teams of the web platform that we are developing.

Our design is the following:

Customer Service to our Platform -> internet -> digitalocean cloud -> Firewall -> Loadbalancer -> Droplets Web Platform Backend -> Process Request -> Response Droplets Web Platform Backend -> Loadbalancer -> Firewall -> digitalocean cloud response output -> The client receives a response to the request.

Due to the previous design we have the following questions:

1. The Firewall must have a public IP? I ask and configure our domain with this ip so that the requests of our clients reach this.
2. What is the limit of the transfer of incoming traffic (From the internet) and outgoing traffic (response from backend droplets passing through the loadbalancer) of the Firewall?
3. Because the loadbalancer will receive the traffic sent by the firewall, it can redirect the traffic to the local IP’s of the droplets? Is this traffic charged?
4. Any suggestions on this design since the platform we are building is transactional and we must guarantee high availability of the 7X24X365 service.

Thank you,

Cordially,

Luis Cardona