There's several controllers available. The best solution would be to get an Apex, but ... most of you guys are too cheap to spend more than $5 at the auction, let alone $800 for an Apex. 🙃
Devices like the Inkbird, the AutoAqua, and even the controller from JBJ all get the job done. You just set them to turn off at like 84°, set your heater to 80°, and it gives you 4 degrees of wiggle room. They range in price from about $30-80, and you're gonna get what you pay for. Keep in mind that they can, too, fail. Probably the most common way is their probe: they use a little probe, like a remote meat thermometer, that sits in the tank. It can fall out of the tank, in which case, it'll measure the room temperature, not the tank....
Of course, you also have to worry about the heater getting stuck on, and not knowing because the controller is now controlling it.... So watch your temperature.
There's no way a meter will tell you if a heater is about to fail or anything. They all work with a simple strip thermostat inside... eventually, these fuse, and the things get stuck on. It happens unpredictably. The more the device switches on and off, the more likely it is to happen -- and also if it just plain stays on for long periods (which runs contrary to the advice of multiple heaters: if you have multiple heaters that stay on longer than a single large one, they're more likely to fail).
Oddly enough, and I've never had this explained to me... but the technology in the heater isn't so different from that in an older home's thermostat, or if you just bought an older style thermostat. Yet, I've NEVER had a home thermostat fail .... Has anyone?