The min/max helper can do sums but if any of the devices is unavailable, the whole helper turns unavailable. This is really irritating if you for instance are tracking current energy for a bunch of lights, and one of the lights gets disconnected and say replaced for a month with a holiday themed light and all of a sudden you have no energy tracking for lights for a whole month.
I just want the helper to have the option ignore the unavailable devices and do the sums of the available ones
If it did what you asked it would cause issues with the energy dashboard.
If you only add the available entities the total can go down after one becomes unavailable. You will get incorrect readings when the sensor is added back and the total goes up again. Going from unavailable back to the full total of all the sensors will not cause this issue.
EDIT: you should track your lights separately if you are removing them seasonally.