OPNSense deprecated (is deprecating?) the included ISC DHCP server and now has the Kea DHCP server as standard. I migrated to from ISC to Kea in OPNSense and it was relatively painless, and it's been working well since. No complaints here, but my setup is pretty vanilla.
I can't comment on the DNS integration, but I might look a bit deeper because it sounds useful.
ISC shut down the DHCP project in 2022 (and afaik, nobody has taken it up as a fork), so it's less of a OPNSense decision and more of an ISC decision. Nothing is stopping anyone from continuing to user ISC dhcp for a long time, but people are reluctant.
It sounded like they were encouraging dnsmasq for home use. I migrated to that successfully. My DHCPv6 is working flawlessly now whereas I was never able to get it running smoothly/persistently on ISC.
I understand Kea has more features so I'm a little curious what I'm missing.
I, too, was under the impression that Kea is now mostly out and they're going the dnsmasq route.
There were open issues about some basic features with Kea, too: https://github.com/opnsense/core/issues/7475
I can't comment on the DNS integration, but I might look a bit deeper because it sounds useful.