You need to make two distinctions. "Doneness" and tenderness are two different effects. Doneness refers specifically to the condition of a meat when it is at a particular temperature. For instance, "rare" beef for most people is at 120-130F. Well done is 160F or above. What is important to understand is that the rare beef will be rare, that is cold red, at the moment it hits 120F and will remain rare, no matter how long you leave it at that temperature. In sous vide cooking, for instance, the meet is vacuum sealed and immersed in a water bath. If that bath is 120F, the entire thickness of meat reaches that temperature. You can leave it in there for days, and it won't be other than rare.
Tenderness, on the other hand, it a quality that is a combination of the character of the meat and how long it is cooked. "Tender" cuts, like ribeye, are tender by nature. They are part of a muscle that does little work and has little connective tissue. In fact, you will ruin them and make them tough by cooking too long.
The "tougher" cuts, sirloin, chuck, etc., have more collagen in the form of connective tissue and between muscle fibers. They start out tough, but we love them, because they have better flavor than the tender cuts. But we can cook them a long time, and that time breaks down the collagen into gelatin, and that makes them tender, and the gelatin gives them wonderful mouth feel. The cooking temperature doesn't have to be high. They just have to cook a long time.
We normally experience those "tough" cuts well done, because we do things like braise them, which means essentially at 212F, the maximum temperature of the braising liquid, in a very hot oven, for a long time. Or stewed them, as you did.
Seen the Outback Steakhouse ads that boast sirloin steaks? What's up? Sirloin's tough. How do you present a medium rare sirloin. Restaurants today commonly keep steaks to be served medium rare in a sous vide bath at, say, 135F. They have been in there for hours, and are therefore tender. And they will never be more than medium rare. They take one out and put some grill marks on it. Medium rare AND tender, in a notoriously tough cut of beef.
Your choices for fork tender are (1) buy filet or (2) cook sirloin tip roast for a long time. The filet demands a very hot pan or grill to brown it without overcooking. That's why you have to buy them an inch thick or more for home cooking. The restaurant can handle thin cuts, because they have massive heat to work with.
You can, though have your meat and eat it too. You can precook your stew meat in a slow cooker and refrigerate it or freeze it and use it in the curry when you want a quick meal. The long cooking will make it tender. One caution. When you take it out, you want to quickly cool it to below 40F to prevent bacteria growth. This is unlikely to be a big problem, because it is pasteurized (NOT sterilized) in the cooking. You don't want to throw how meat in a refrigerator and draw the heat down in the compartment. But you can put it into a zipper freezer bag and cool it in an ice bath. To make it cool efficiently, put the meat in the bag, zip it nearly closed, and carefully immerse it in a bowl of water, but not up to the open part of the closure. This forces most of the air out, and you can then zip it closed and put it in the ice bath. Not having the air will make it cool quicker and will help prevent freezer burn.
This will keep for days in the refrigerator or almost indefinitely in the freezer, and you if you put one meal worth of meat in each bag, you can pull one out and thaw it in the fridge the day before and have it ready to cook with.
