OpenWISP looks to be the furthest along, though right now I think it only supports OpenWRT https://openwisp.io/docs/index.html

There is some OS tooling in the SDN realm, like Stratum[1] for example, or a P4 board for the serious. But the hardware behind it isn't cheap.

I wish router for personal use were as "easily" programmable as an OpenFlow compatible equipment with a external controller. Even if you need some extra tooling to reach all the feature of RouterOS, like a compute node for the DNS. I don't know if this kind of evolution will ever reach the consumer space.

[1] https://opennetworking.org/stratum/

I've always wondered if one could use SNMP for this.

Doubtful. Some basic stuff is supported across almost all devices (interface names, speeds, status,...), but more detailed info varies widely between vendors, their OS versions and devices. SNMP SET support is mostly a joke and not worth the trouble. Better use whatever API each vendor came up with.

I know that the Asuswrt integration with Home-assistant lets me manage devices which is kind of cool, but I too would love a little deeper access via a 3rd party app. Most of these things use web scraping or ssh to the device though, not an actual API as very few routers give access to one.