Configuring IPv4 router advertisements as alternative to DHCP default gateway?

[eas]

My fiber connection becomes unreliable during periods of sub-freezing weather. My telco hasn't been able to fix it thus far.

My client devices (primarily Macs and iPhones) have ways of, individually, automatically failing over to alternative connectivity options, but they won't do it because as far as they are concerned, my Opnsense box is still the best way to get to the Internet at large.

It seems to me that if there were a way to instead provide clients with the route to 0.0.0.0 via automatic route advertisements instead of DHCP, the clients could automatically fail over when the connection goes down and recover when the connection comes back up.

It seems like this is already in place for IPv6. Is there some way to set it up for IPv4? Is there a plugin I need to install and configure?

Any guidance would be appreciated. Thanks!

Update: I found FRR, which seems like it would do what I want to do on the OpnSense end. Unfortunately I can't find any evidence that MacOS natively supports gateway discovery by any means other than DHCP.