The problem with your beverage weight variance doesn’t lie with your volumetrics - they are functioning as intended (especially now after being replaced!). Instead, your Mythos, and the 20°C swing in grinds temperature is the cause.
Provided there is a steady flow of coffee going through the Mythos, and it was either left on overnight or allowed 30+ minutes to “heat” up, the grinds will exit at around 30°C during your seasoning shots, move to 35°C after around 10 or so constant shots, then gradually climb to 40°C as you get busier. At your peak time, once the burr heat has fought past the fans constraining or cooling the grinds, you’ll start hitting 50 - 52°C, and up to 55°C if you’re getting hammered.
This change in grinds temperature changes the grind profile (if grind setting is kept constant, as you heat up you’ll have a decrease in fines, yet they will increase in size) and also changes the grind profile behavior in the basket. This all leads to a curve in resistance of the puck to pressurized water. At the start of the day you’ll have little resistance, you’ll grind fine to increase it, the burrs will heat up, eventually you’ll over shoot the mark, grind coarse, then at your peak time you’ll either stay coarse or grind finer. The set amount of water you have programmed for your volumetrics will either have limited access through the puck at high resistance, or essentially flow straight through the puck at low resistance. This manifests in you hitting your target yield (at the start of the morning, low grinds heat) or being over your target (during your peak, high grinds heat). This is assuming you have a constant supply of coffee through the grinder during the day. As their are natural fluctuations in the ebb and flow of customers, there’s also an ebb and flow in grinds temperature, leading to fluctuations in resistance of the puck.
What I do to combat this (we don’t have scale integration with our Opera yet, so rely on volumetrics - and we are very busy: 50-60kg over five days, majority between 7-11am) is to use volumetrics in tandem with intermittent scale use. I change the grind based on my yield - if it’s over, I increase resistance and grind fine, if it’s under I decrease resistance and grind coarse. I don’t worry about shot times, I let that take care of itself, allowing it to range from a ~28 second shot at the start of the morning (low grinds heat, high resistance) to a ~38 second shot during peak times (high grinds heat, low resistance). I’ll check maybe every third or fourth shot off the line, adjusting the grind setting according to the weight.
I was in the same boat as you, frustrated with what I saw as inconsistent volumetrics. In the end I found it was the grinds temperature that was causing the inconsistency, and this was simply a byproduct of their functionality - burrs get hot, the heat affects the grinds. It’s taken a change in mindset, but this has improved workflow and consistency massively. Hope this helps.