Your central CA could be your AD, one of your pfsense firewalls, some public or any other CA you want to use.
Central CA for all certs you sign is a good idea, be it user certs for openvpn, AD, internal websites, etc. more than likely this should end up being the one you use for AD.. most likely the MS built in one.
The only certs you really need from public, are ones that will be used by public with machines that you do not control and can not trust your own internal CA. If you have a website that is accessed via public, then you need to use a public CA that users browsers auto trust. If the site or vpn is only access by your machines, or users that you can give the CA cert to trust. Then internal works great, centralizing that makes for easier management and control.
While using the CAs on pfsense does make it easy, the interface is pretty clean. But if your going to manage lots and lots of certs it might get a bit hectic.. If it was me, I would most likely leverage the AD CA, since you pretty much use that for all your machine certs as they join your domain anyway. Might as well just leverage it for all your internal use certs.