Thank you @Zxurian for your elaborate findings! I was also caught up on the logs being spammed by "br0: requesting DHCPv6 information" and you brought me to check a few things. In my case it was actually easier, as I actually don't run a DHCPv6 service in my network.
All my OPNsense settings are the same as yours. What I had to change was:
Services > Router Advertisements > LAN > Router Advertisements: Unmanaged
I had it on "Stateless" before. As far as I understand that told clients they should do SLAAC as well as make DHCPv6 requests. "Unmanaged" tells clients that they should only do SLAAC (and I guess that there is no DHCPv6 server available).
This is the description of the options inside of OPNsense:
No my log is not spammed anymore! 🥳 For those interested, this is how the log looked like directly after the change:
Jul 17 11:13:56 NAS dhcpcd[1027]: br0: requesting DHCPv6 information
Jul 17 11:14:01 NAS dhcpcd[1027]: br0: fe80::5054:ff:fe2c:6e5f: no longer a default router
Jul 17 11:14:01 NAS dhcpcd[1027]: br0: deleting default route via fe80::5054:ff:fe2c:6e5f
Jul 17 11:14:01 NAS avahi-daemon[9346]: Withdrawing address record for 2001:9e8:4c7e:6200:6ebf:b5ff:fe02:897b on br0.
Jul 17 11:14:01 NAS avahi-daemon[9346]: Registering new address record for 2001:9e8:4c7e:6200:6ebf:b5ff:fe02:897b on br0.*.
Jul 17 11:14:01 NAS avahi-daemon[9346]: Registering new address record for fe80::6ebf:b5ff:fe02:897b on br0.*.
Jul 17 11:14:01 NAS avahi-daemon[9346]: Withdrawing address record for 2001:9e8:4c7e:6200:743:303f:90a6:531f on br0.
Jul 17 11:14:01 NAS avahi-daemon[9346]: Withdrawing address record for fe80::6ebf:b5ff:fe02:897b on br0.
Jul 17 11:14:01 NAS avahi-daemon[9346]: Registering new address record for 2001:9e8:4c7e:6200:743:303f:90a6:531f on br0.*.
Jul 17 11:14:01 NAS dhcpcd[1027]: br0: adding default route via fe80::5054:ff:fe2c:6e5f
Jul 17 11:14:01 NAS dnsmasq[14716]: reading /etc/resolv.conf
Jul 17 11:14:01 NAS dnsmasq[14716]: using nameserver 192.168.1.1#53
Jul 17 11:14:01 NAS dnsmasq[14716]: reading /etc/resolv.conf
Jul 17 11:14:01 NAS dnsmasq[14716]: using nameserver 192.168.1.1#53
Jul 17 11:14:01 NAS dnsmasq[14716]: using nameserver fe80::5054:ff:feaa:33d1%br0#53
Jul 17 11:14:06 NAS dhcpcd[1027]: br0: no advertising IPv6 router wants DHCP