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.
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.
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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.