The firewall conversion went smoothly thanks to the migration wizard, which is virtually foolproof and easy to use even for a layman.
I use OPNsense in my home network and therefore decided to switch from ISC DHCP to Dnsmasq DNS & DHCP.
As a layman, it took me some time to understand that in my case, Dnsmasq DNS & DHCP should "only" replace ISC DHCPv4/6 and not Unbound DNS as well (Dnsmasq DNS & DHCP is "only" used for resolution in the local network).
That's why it took a while to get adguard up and running again.
(Luckily, there are snapshots :-)
Once that was clarified, I uninstalled the ISC plugin and everything is working wonderfully – thank you very much
Oh, and this guide (https://homenetworkguy.com/how-to/migrate-from-isc-dhcp-to-dnsmasq-or-kea-dhcp-in-opnsense/) helped me a lot: