Ok, first for the trigger.
You don't need trigger conditions.
You can use a recurring flow, once per month on the 15th.
Then you use your first condition:
If your condition is met, you can do what you need to do
If not, you have the flow wait for a day (until the 16th)
Then you have your second condition (same as the first one)
But your second option here is to terminate the flow.