Place attribute "AmountToDate" in Entity1, still as calculated field.
The calculation microflow for field "AmountToDate" will sum the amounts for the relevant Entity1-Objects.
There is no data-overhead since calculated fields don't get stored.
As you have a list of entity 1, performance wise it would be better to use a list operation to aggregate the "AmountAdded" attribute instead of iterating on it in a loop.
As Tim said, as this is a calculated field the attribute will only "change" when the entity is being used somewhere, whether that is in a microflow or showing it on a page.