Hi CayerA,
Well I think you have come quite a long way on completing your project. Congratulations.
One thing I think you should stop doing is using the fire stations or the fire station response areas as your clip polygons for the grid. When you do this the grid will always label at the clip polygon boundary.
Instead use the grid feature layer, attach it to the map extent, and turn labeling on. See what happens. If you don't immediately get the result you want, read up on grids and graticules. I can't drive the boat here for you, but I can point you to instructions which will help you find your way. For the moment let go of your focus to get to the end goal, be curious about what happens when you press that button or turn that switch on, and play with the program. https://pro.arcgis.com/en/pro-app/latest/help/layouts/grids-and-graticules.htm . Just remember to save your work periodically. This frees up memory to do other things. It also saves your work just in case the program freezes or crashes. If you don't like the monster you have created, exit the program without saving changes or use the backup button until you're back where you started.
As far as making a map series goes, I recommend you pursue this. That way you have to make just one map layout. Follow the directions in https://pro.arcgis.com/en/pro-app/latest/help/layouts/map-series-tutorial.htm Use your fire station response areas as the series focus. In the properties section of this polygon you need to look at the page query and select "must match". You can also make a copy of the response area polygon, rename it "adjacent response areas", give it a different shade, and mark it "must not match" in the page query. I think that's the wording. That way your map series will contain the other response areas, but will not be the focus.
Another thing you can do with the map series is with the items on your legend. Select legend properties and noodle around in there until you find " only label items which are on the map extent". That way your map legend does not contain items which are not on it.
Let us know how you're doing. I recommend you look at esri training courses https://www.esri.com/training/arcgis-online-training/ and GIS classes at universities. Like at esri training, some offer on-line courses where you will interact with your teachers and classmates. That way it's not such a lonely rat in a maze pursuit.