I use my APEX and two temperature probes as backup. If my temp goes over 80 I turn off power to the heater and if the temp is less than 78 I turn power on.Sorry for your loses. Most of what survived will most likely recover. Go over the rocks carefully looking for dead organisms and remove them, or just keep them in a fallow environment and watch for dead organisms.
When you have a major temperature change event (either way), bring the tank and inhabitants back to temperature slowly.
Unfortunately heaters are the single biggest cause of system crashes. There just are no fail-proof heaters. Eheim heaters are very good and fail less than many others. There are threads and threads on heaters and redundancy. You can go crazy with redundancy. Failing ON is the most common heater failure so a simple external temperature probe connected to a cutoff switch is a 90% solution. I know someone makes one but I haven't been able to find it now that I want to find it. I'm not really big on using a heater controller to control the heater. I'd rather have a simple voltage cutoff if the temperature goes more than a few degrees over the set temperature. That said Inkbird is very popular and anything RANCO-based is probably as good as it gets.
Give your tank and what's left time to recover. More has survived than you will believe.