Do any of you happen to know if it is possible to setup PfSense as DHCP/DNS in a way that it would not break AD, but would still allow internet access in an instance that I shut all of my AD/DNS/DHCP servers down?
Yes, and quite simple. Setup pfsense as your router and configure it to run a DNS server, which will use external DNS servers. Then your internal windows DNS servers can simply point to your pfsense router as a resolver for anything outside your AD domain - so your domain controllers will handle all the DNS for your domain, and pfsense will handle everything else.
It also allows greater flexibility internally, since you might want to segregate a few networks away from your AD stuff. This way someone on a guest network wouldn't need to be given access to your domain controllers, they could just be pointed straight at the pfsense DNS server.
As I understand it, DHCP does not have to be handled by Windows, but AD does require DNS to be handled in its sphere for all of the lookups that AD requires. But that doesnt mean that it has to be the ONLY dns.
DHCP can follow a similar model (yes, AD-integrated DHCP can be done outside of MS DHCP servers but it's a bit of a PITA); your segregated AD networks get their DHCP from the windows servers, all your other networks can get it from DHCP servers running elsewhere (like the pfsense box).
Personally I use dnsmasq as my DNS server since it also handles DHCP automatically (and will register local DNS names for DHCP-issued IPs without having to muck around with stuff like TSIG) and a samba4+bind9 setup for my internal active directory DNS. dnsmasq thus handles all DNS and DHCP duties for everything not on the domain networks, and samba+bind handles everything on the internal domain networks, referring up to dnsmasq when it needs to query t'internet-based DNS (although none of my domain machines have direct internet access).
S'about as KISS as you can get whilst still maintaining network segregation; you're not using managed switches so chances are you might have no interest in network segregation at all, but it's still easy to do with dumb switches - you just add more NICs to pfsense and get it to handle things through dumb switches instead - not as neat as a managed switch but cheaper for experimentation purposes.