My memory is a bit blurry, but IIRC with `nodetach` option pppd would fit "simple" category or its derivatives well.

Teaching it to notify via dbus is as trivial as passing a bunch of `connect /usr/lib/networkd/ppp/ip-down`-type options or writing a simple plugin library to do dbus messaging or `sd_notify` calls. Available plugin API hooks should allow for anything necessary to have a good understanding of pppd's state and provide a good amount of control over it (link state changes, authentication, IP address negotiation, idling etc) when necessary: http://ftp.samba.org/pub/unpacked/ppp/PLUGINS

I'm almost certain there's no need to write a whole new pppd from scratch.