The correct setting for SLAAC-only is "Unmanaged". You should disable the DHCPv6 server first.Does radvd run? If so, try restarting it. Changes in the interface might not be seen unless the daemon is restarted (this should be O.K. later).Also, try to reduce the two intervals to 200/600 seconds.If prefixes do not get distributed, have a look at /var/etc/radvd.conf and see if the prefixes are in there.
network: version: 2 renderer: networkd ethernets: eth0: dhcp4: yes dhcp4-overrides: use-dns: true use-domains: true dhcp6: no accept-ra: true ipv6-privacy: true optional: true mtu: 1500