Here are a couple that I have had good results with. Ember is right, this kind of cooking: thickness is far more important than overall weight.
If you had a tough time cutting it, regardless of whether it was properly cooked, sounds like it might be time to sharpen your knives. I’ve yet to come across a cut of meat, raw or otherwise, that doesn’t yield easily to a properly sharpened knife.
THe other thing you can do with a rump or eye of the round roast is to cut the meat paper thin for sandwiches. Thisis what I do when I do not have time for a longer cook. Some crusty French bread and Au jus make a great meal.
Try the Poly Science Sous Vide Toolbox app available on iOS. Allows you to program in the thickness, the shape, the type of meat being cooked, the cook temperature, the initial meat temperature, (ie. Room, home fridge or commercial fridge). It also tells you when your meat has been pasteurized and what bugs (Salmonella, EColi, Listeria, etc) have been eradicated. It’s probably the most comprehensive Sous Vide app I’ve found to date. Unfortunately the developer needs to update it in order for it to work with iOS 11.
I read on other sous vide sites that tenderizing tough beef is best at temperatures of at least 55C/131F or higher. I cook 2 to 3 inch-thick cuts of rump, shoulder, bottom round or tri-tip (bottom sirloin) at 55C/131F for about 18 hours, and they come out pink and very tender, without the mushy-stringy texture that results when cooking tough beef for too long.