DHCPv6 ntp server not working

Installing and Using OpenWrt
8

Sorry. I have no guide or the like. I can only recommend to use https://openwrt.org/docs/guide-user/base-system/dhcp and make the best out of it and pay closer attention to list dhcp_option which translate to dnsmasq config syntax --dhcp-option= (See https://thekelleys.org.uk/dnsmasq/docs/dnsmasq-man.html).
For a full list of dhcp-options, see: https://www.iana.org/assignments/bootp-dhcp-parameters/bootp-dhcp-parameters.xhtml#options
If a name is supported by dnsmasq, or not, you can check in the source, see this list: https://github.com/DNSFilter/dnsmasq/blob/master/src/dhcp-common.c#L498

For IPv6: The same.
If you do not find it in the openwrt wiki page, check the dnsmasq man page. And the IANA assignments are located here: https://www.iana.org/assignments/dhcpv6-parameters/dhcpv6-parameters.xhtml#dhcpv6-parameters-2

Config wise, I can provide you with a snippet I have in use:
(I can not recall the details, but I assume in /etc/config/dhcp was some stanza for odhcp6, which I have removed completly.)
I have this minimal config. What was once set in this file but is a default regarding to the wiki, was removed, so I should have only values left, which differ from the default. Please check yourself...
For each VLAN I have more or less the same settings, see below:

config dnsmasq
    option  leasefile           '/tmp/dhcp.leases'
    option  localservice        '0'
    option  fqdn                '1'
    option  nonegcache          '1'
    option  cachesize           '1500'

config dhcp                     'vlan17'
    option  interface           'vlan17'
    option  domain              'srv.mgmt.home.arpa'
    option  local               '/srv.mgmt.home.arpa/'
    option  start               '32'
    option  limit               '215'
    option  leasetime           '12h'
    option  dhcpv4              'server'
    list    dhcp_option         'option:dns-server, 192.168.0.1'
    list    dhcp_option         'option:domain-name, srv.mgmt.home.arpa.'
    list    dhcp_option         'option:ntp-server, 192.53.103.103'
    list    dhcp_option         '100,UTC'  # PCode
    list    dhcp_option         '101,UTC'  # TCode
    list    dhcp_option         'option:domain-search, srv.mgmt.home.arpa.'
    option  dhcpv6              'server'
    list    dhcp_option         'option6:dns-server, [fde6:a09a:b373::1]'
    list    dhcp_option         'option6:domain-search, srv.mgmt.home.arpa.'
    list    dhcp_option         'option6:ntp-server, de.pool.ntp.org.'
    option  ra                  'server'
    option  ra_slaac            '1'
    list    ra_flags            'managed-config'
    list    ra_flags            'other-config'

Edit: Pay attention that the code block has a scrollbar!