I've only been in the hobby for three years , used a few different salts, Red Sea, hw reefers, EVS, brightwells, water in container, pump , heater , then salt, don't know how i could have done it wrong, I think I can blame the batch of salt!!
Due to your experience, blaming the salt makes sense, but its not fair, especially because hw marinemix is one of if not the purest salt mixes available: synthetic salt with zero impurities or anti caking agents unlike most salts. If you had issues, you have to question the mixing method as well.
Different salts need to be mixed differently. I can tell you right now one mistake you made was using a heater. I recommended mixing at room temp then adding a heater and bringing up to temperature if necessary. Having said that, I do 15% water changes and never have to heat my fresh sw because it doesn't effect my tank temp more than .5 degrees max. Also too strong of a powerhead can cause precipitation or mixing too long can cause precipitation.
Just because mixing one way works for one salt, doesn't mean it will work for another. Hwmarinemix is a synthetic salt produced in a lab and is by far the most consistent salt I have ever used from batch to batch. I never have issues and it's because I mix it gently without heating for a short time. Imo it was not the batch of salt for those reasons.
Another example, I used to use red sea blue bucket. If I heated while/before mixing I would get brown crud on the sides of the brute. This is anticaking agents and other impurities in the salt mix (hw marinemix does not use/need anticaking agents but may still precipitate calcium carbonate if heated during mixing or if mixed for too long). If I mixed my red sea blue bucket the exact same way without heating, no brown crud.
Coral pro salt precipitates like crazy if you mix it too long due to high alk/calc/mag, they are all different!
Use whatever works for you though! Plenty of TOTM use plain old IO.
Hwmarinemix reefer is the best salt for me due to its lack of impurities and anticaking agents, outstanding track record in public aquariums and aquaculture facilities, and the fact that the levels are very close to what I keep my tank at anyway.