Sorry to hear. That really sucks, but please don't give up. I've read your posts over the years and seen what you've kept and sold and you are far too knowledgeable and capable of a reefer to give up the hobby for any amount of time IMO.
I can't really think of much advice to give personally, but I can say this.
I know very few people think to check Ammonia, Nitrite, and Nitrates after the tank has cycled, especially in a mature tank, but what I do personally, is every week when I do my water changes, before I do the WC, I bought those cheap 5 in 1 or whatever test strips that check Ammonia, Nitrite, and Nitrate amongst pH and I think water hardness IIRC (i only pay attention to the first 3 things on the strip) and even though it's far less accurate than a liquid test, it does alert me if something funky is going on.
They're so super cheap that I can afford the $20 or so for a years supply and the peace of mind is nice. It's good to see if any levels are high before my WC. Then if a reading is slightly above 0ppm, i'll know to take head and i'll do a liquid test with the more expensive test kit and double check readings to verify. Maybe it's too much work and I go overboard, but it makes me feel more stable.
I'd absolutely recommend you get a digital controller like an Apex, RKL, or whatever brand you fancy however. It'll let you know through temp swing if a heater is turning on or not, and also they have built in circuit breakers so that in the event of a heater doing as yours did (I could be wrong) but I would imagine that would flip a circuit and disable the heater. Also, when I do my weekly maintenance, I check the conditions of all equipment and wiring. I also like to keep my products up to date and anything I feel is cheap or outdated I like to replace. I too had a Visitherm crack and break after it got stuck on. Luckily it way only an overnight in a saltwater mix bucket.
I know in your case the heater doesn't light up so it's tough to see if it is on or not.
And I want you to know that I am in absolutely no way trying to say it may have been any percentage your fault, so I hope I didn't come off that way.
Best of luck, and if you need any help tht I may somehow be able to provide from this side of Massachusetts, LMK.