Will an in line water regulator fix my tankless water heater trade off between pressure and hot water?
You need a water heater unit with a higher flow rate to bring the water temperature from 40 degrees to 120 degrees. Here in the Pacific Northwest, with year round water temps of around 45 degrees, it is advised to buy a MINIMUM of 7.4 gallons per minute flow rate in a tankless, whole house unit. At 2 or 3 GPM, your purchases are incapable of doing the job. (A 75° F shower is too cold for most of us at 98.6° body temps).
Flow rates out of showers and faucets have little to do with this. They are regulated by the government to save water from, for example, around 1.0 GPM at faucets to 2.2 GPM for a shower. I am referring to the flow rate of water through the tankless heater.
Many people think they can buy and install what is essentially a point-of-use tankless water heater (serving 1 shower or sink, for example) for whole house use to save money and get into trouble.
Is your fixture intended for whole house usage? (Serves all water heating needs in house, showers, sinks, dishwasher etc.) The answer to this question will guide my additional advice.