The way I use to do this takes work on your part, but will accomplish the goal.
I create one new field, Savable. I use a boolean. This field does not have to be visible within your form.
In each field that has a validation, I add to the validation -> and Savable is not equal to true. (if Savable is True, validation wont require anything)
In the Save button, add a rule before your submit rule, Set Savable to True. The rule should set the field Savable to true. This will turn off all of the validation and allow users to save the form.
In the rules for opening the form, add a rule, Set Savable to false. The rule should set the field Savable to False. This will turn on the validation every time you open the form.
The longest part will be adding the extra validation rule.