I did some further testing and found a configuration that seems like it is working for me. Both MacOS and iOS just keep the old IPv6 SLAAC addresses longer than they should. But with ifconfig en0 i was able to check that at least MacOS deprecates the old IPv6 addresses immediately. One thing i noticed is that i have to restart my MacBook (or at least turn off and on WiFi) after a router restart to get it working properly.
Excerpt of /etc/config/dhcp
config dhcp 'lan'
option interface 'lan'
option dhcpv4 'server'
option dhcpv6 'server'
option ra 'server'
option ra_mininterval '30'
option ra_maxinterval '100'
option ra_useleasetime '1'
Maybe this is helpful to someone having the same problem.