Yes, the reheating isn't really a safety issue because it happens over a few minutes. There's not enough time to create a bacteriological problem assuming one wasn't already present.
Rather, assess the food before reheating. how old is it? Check smells, textures and appearance. If any of those are off, toss it. George Carlin used to say that leftovers made you fell good twice. Once when you saved them and thought you were saving money. AGain when you threw them out and thought you were saving your life.
Some foods are better as leftovers; stews and soups tend to improve. Others tend to degrade; rice dries out, pasta clumps, salads wilt and so on.
While the freezer is good for longer term storage, it exacts a toll many foods. The flavor balances shift often becoming weaker, a few do become stronger, garlic and spicy hotness. It might be that those flavors are less affected and only seem stronger in comparison to the other flavors that have faded.
Textures also change. Lots of food will weep moisture when thawed.
Foods best for freezing tend to be high moisture content foods. Your casserole is a good candidate for freezing. However, you wouldn't want to freeze it in tupperware for more than a few days. The airspace creates the conditions for freezer burn. For longer storage, you could initially freeze it in the tupperware, then remoev it from the tupperware, wrap it in plastic wrap/foil or both.
Thawing is best done over a day or two in the fridge. This eliminates most food safety issues and I think the food comes out of the freeze in better shape. I prefer this to reheating from frozen as you get more control over the reheat which prevents burns, scorches or dried out ends.
Many prefab foods can be microwaved from frozen. Some of your own leftovers can be done this way. Soup is a no brainer for this technique. Stir it as frequently to help it come back evenly.
But say a stew, it would have meat and vegetables sticking out that could get dried out or over heated. Or meat could render some more fat and overheat a spot on the tupperware leading to a bubbled container. Same with a plastic bowl. Sugars, tomatoes and greasy foods should be reheated in a glass or stoneware container. Those foods can get hot enough to burn themselves or melt plastic.
There is a bit of an art to reheating and there is no one universal technique. Microwave is good for high moisture foods, generally. Do only one normal plate at a time. Too much food won't heat evenly and leads to problems. Most food benefits from a sprinkle of water. This helps prevent it from drying out. Cover the food. There are special plastic vented domes that work pretty well. Get a large one that still fits in your microwave. Plastic wrap is good too though there is some concern over the plasticizers migrating to the food. Put the food in a deep bowl so the plastic doesn't touch the food and you're fine. Poke some vent holes in the wrap. Don't use full power. Most modern microwaves have a reheat setting that works pretty well. Be careful, the containers can get hot.
Rotate the food during reheating if you don't have an automatic carousel. Stir foods that can be stirred.
But sauces rarely reheat well in the microwave. So gravy from thanksgiving, I usually reheat in small amounts on the stove.
You won't poison yourself reheating. That's more of a storage issue or initial cool down problem.