Different anode materials react differently with different water conditions.

Curious, how old is the WH you're replacing?

I'd correct the PH of 6 and what does "low on metals" mean in content and numbers?

When was the last time you had a comprehensive water test done by a certified lab? If recently, what are the test results?

Our well was tested about 5 years ago. Metals lab analyses numbers were all very low, much lower than drinking standards and that includes typical metals like Ca, Mg, Fe as well as several heavy metals. Very low in solids and crystal clear. I can dig up the old report for specific numbers if that helps.
The water heater is now about 40 years old and is the one I have another thread on about draining due to clogged drain valve.

I assume pex doesn't require grounding? or that it's even possible?

Use the same type of anode you had before.

You have done the test ("water heater is now about 40 years old") use the results.

Non-conducting pipe cannot be grounded (bonded actually) but if there are any large metal pipe sections they should be bonded, especially if they are likely to become energized.

Small pipe sections and faucets are not likely to become energized.