The template sensor will update itself when any of its sources are updated. Which in your case will be at the top of every minute per HA’s own clock (due to now() being a source, unrelated to the fact that you’re using the .minute property) and then also every time the rain sensor is updated, which according to you is once every minute (but unlikely to be aligned with the top of every minute according to HA’s clock). So the sensor will be updated twice a minute, but not necessarily every 30 seconds. For example it could be 10 seconds between updates, then 50 seconds for the next update, then 10 seconds, etc.
That said, only updates that are different will be recorded. The update at the top of the minute will always be different, but the next update will only be different if the rain gauge has a new value.
The statistics sensor should update when the source sensor gets a new value added to the database, and also when a source sensor data point ages out of the max_age window.
If you are confident that your rain gauge will provide an update on a consistent time interval, then you don’t need to enforce that time interval in your template sensor attribute and you can remove the reference to now(). Instead, you could do something simple like set the attribute to 1 if it is currently 0, otherwise set to 0. This would just give you an alternating 1 and 0 on every update and therefore force the sensor to record the data whenever an update is sent.