Curious, I have been using epoxies for more than 40 years and have read many articles on epoxies in for instance Adhesives and Sealants magazine, and I have never once seen or heard of any epoxy whose properties do not depend on mix ratio; although in many cases exact mix ratio is not critical.
"Epoxy" is not an adhesive, it is a family of adhesives that includes many different specific chemistries. All high strength epoxies are multi-functional, that is, they are a blend of two or more of the basic epoxy chemistries. In any critical application it is foolish not to discuss your specific requirements with one or more epoxy formulators to obtain a blend closely optimized to your needs, and then to qualify the materials and application process with testing. Master Bond is one supplier of specialty epoxies with good tech support.
On the other hand, for hobby applications, you can get good variation in hardness with mix ratio with any nominally 5:1 ratio marine epoxy. These epoxies are normally available with slow or fast hardeners in order to allow reasonable working time in cool (use fast hardener) to hot (use slow hardener) conditions. More hardener results in more flexibility with lower strength and hardness. I mostly use "The West System" with pump dispensers for easy measurement - one stroke of each is 5:1, for small amounts I count drops. With the addition of fumed silica (available from the same marine suppliers as the epoxy) it can be thickened into a non-running paste very easily, or with the addition of fine wood dust as a filler you can make epoxy foam.
BTW, by far the most common reason for lower than normal strength epoxy is inadequate mixing, especially in the filled, pasty formulations. It is best to consider the point where the epoxy appears to be completely mixed to be the half way point in mixing. If you actually measure the strength or hardness of the cured epoxy the additional mixing will almost always result in better and more consistent properties.