CentOS - Oct 2021 - IPv6 routing preference with multiple default gateways

I've got a primary and backup CentOS 7 gateway, each with two interfaces, 
connected to my LAN and my fiber gateway. The default gateway (the fiber 
box) is set explicitly in ifcfg-eno2 with IPV6_DEFAULTGW. Using "ip -6 
route show" I see two defaults, the static one and the one advertised from 
my other CentOS7 box via the internal interface (eno1), both with metric 
1024 and preference medium.

default via 2001:1890:1837:5b00::1 dev eno2 metric 1024 pref medium
default via fe80::100 dev eno1 proto ra metric 1024 expires 293sec hoplimit 
64 pref medium

How do I set the IPv6 router advertisements on the LAN side to advertise a 
higher metric or lower preference so there's never ambiguity that the fiber 
box has precedence? I can't find a setting in the man page for radvd to set 
the metric or preference for the LAN route that's announced.

Additionally, I think I need to announce a route on the WAN interface 
(eno2) of lower precedence than the static route so the fiber box knows 
that the two CentOS7 boxes are proxies for my LAN, but the two boxes don't 
try to route to each other?

This is for C8 but it's useful for the concepts, like that a lower metric 
has a higher precedence:

<https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/configuring_and_managing_networking/managing-the-default-gateway-setting_configuring-and-managing-networking>

Some radvd examples that lack an example of how to set metric or preference.

<https://tldp.org/HOWTO/Linux+IPv6-HOWTO/ch22s04.html>

Ugh. It looks like I need to read through the neighbor discovery RFC to 
understand how this works:

https://datatracker.ietf.org/doc/html/rfc4861

I did discover that the radvd.conf page includes an entry for 
AdvDefaultPreference low|medium|high so I can at least set the preference 
of the backup C7 box to low.