So if I understand this correctly, is my new CRS309 with updated firmware, using RouterOS in bridge mode which essentially means it's a dumb switch out of the box?
RouterOS is a highly versatile when it comes to configurability and functionality. There are a few pre-defined configuration profiles which can be selected in Quick Set mode. So yes, selecting Bridge mode makes your CRS more or less a dumb switch, as much as it's dumb when running SwitchOS with default configuration.
If you want to make it a tad less dumb, you can do it both in SwOS and ROS (even if you started off from Bridge mode).