Problem to disable only AdvDefaultLifetime and not AdvRouteLifetime?

Hello there,

for many trials, google searches and readings of documentation, i couldn't figure out how to achieve the following:

On a non-normal eth interface (that is, the other eth interfaces are bridged together normally and route to the internet normally) which is on a separate bridge interface, i would like to not announce this router as the ipv6 default gateway.
however, it still should normally announce the route to the delegated prefix.

when i set dhcp.NONNORMAL.ra_lifetime=0 in uci, then the ipv6 router advertisements arriving at my host contain (according to radvdump) the global option AdvDefaultLifetime=0 (indicating that this router is not a default gateway) and the on-link /64 prefix, and also as expected the route to the /56 delegated prefix. however, the route to the delegated prefix has option AdvRouteLifetime=0 (indicating, that this router is also not the router for this delegated prefix).

when i don't set dhcp.NONNORMAL.ra_lifetime (thus, the default behaviour), then the route to the delegated prefix is announced normally, but also the global option AdvDefaultLifetime is set to a high value, announcing this router as the default gateway.

but i would like to announce this router as a gateway to the delegated prefix, and not announce it as a default gateway.

in radvd (https://radvd.litech.org/) this would be setting global option AdvDefaultLifetime=0, but leaving the announced routes at their default lifetime.
in UCI, it seems, only a single option ra_lifetime ( https://openwrt.org/docs/guide-user/base-system/dhcp ) is available that controls both lifetime values.

Am i missing something?

cheers,

Max