I have used the standard relay based scheme (https://www.melevsreef.com/plumbing/auto_topoff.html) for auto-top off, and it works fine, but my sump has a lot of flow, so there was always a lot of "clicking" before the pump would shut off, and the relay would wear out every few years, so last year designed out the relay with a Triac. The circuit below is mounted on a PCB (3 for ~$8 at OSHPark). The RC on input knocks out the "chatter", so hopefully the Triac will last longer than the relay. If anyone wants the eagle files for the PCB, just PM me your email and will send them to you.