Hello, one of the things that will cause this problem is a mixing valve sized for a lot less than the total water flow resistance in the rest of the system. The technical term you might want to look up is Cv... the explanations are generally arcane and tough to get your head around.
But can give you an example... a gross exaggeration works well, what if I put a 20 inch cast iron mixing valve on a 1/4" copper line and attempted to control water flow with it... you can see that even a microscopic crack open of the valve would fully load the 1/4" tube. so that you would get ON/OFF control at best.
We have determined over the years that the valve seat in a mixing valve needs to be quite a bit smaller than the line size so as to introduce a pressure drop (friction loss) equal to the friction loss in the rest of the piping, controls and heat exchangers.
The odds are high that your mixing valve is too large for the job.
If you down size it, AND your water pump can handle the additional friction loss (that needs to be carefully looked into)... then you will have good control.
That said, assuming the valve port is sized correctly, there could be a range of other issues, but we should address this port size issue first.
Study up on that a bit, get back to me, we can go from there if you wish.
(my background is 51 years in the business, consulting nationally, running my own 27 man shop for the first 30 years of that.)
Thanks for letting me work with you. When we are finished,if my answer has been polite, professional, and thorough,please locate the 'rate your expert button' and click one of the ratings .If you still need help let me know!.
Rating will not increase your charges, nor close the question, but is how I earn my living.