Im looking for recommendations on what CCR ethernet routers are capable of handling a /19 DHCP Pool.

Trying to keep it short and sweet.

Just as DHCP-server? Probably all of them

I had assumed it would be understood that it will need to do fairly basic firewall as well.

Currently my customer has about 5 services per building and they are using the modems as an AIO network. Unfortunately those modems cant handle more than a /24 network. so they have had to purchase a multitude of services for their network.

Why CCR (Whats your bandwidth your forwarding/inputting )? Most RB's will do fine.
Try a RB4011, good bang for buck, or just run a x86 VM of RouterOS.

What are your throughput requirements?