You can turn off grid charging. For your PW’s. Then only enable it. When you want to use it.I have mine set at 50% reserve mostly because of paranoia. I could probably drop it down to 30% most days. My concern is that I think it will sit at 100% for long periods of time. If the Powerwalls were LFP, I wouldn't worry about it. I suppose if I am on free nights, I could purposely set the rate plan such that on an average day of battery usage, that it doesn't have enough time to charge back to 100%. For example: instead of setting it from 9P to 7A, I could set it from 5A-7A or something like that.
But yes. Tesla should be able to do this. My EcoFlow is LFP. I can choose how much I want the batteries to charge from grid. I have it set to stop charging at 90%. Then it will only top off from Solar.