First of all, sorry about your not closing the quoting tag you used. I do that sometimes, and it's so sad to see all your hard work obscured under an italic fog... so I thought I would at least respond in detail to help make up for it.
The charging point you make is a great one. There are some powered mounts that you can plug a phone into, but they require more work to fit... I prefer a general mount you can use with separate power so that as I change phones I can maintain the same mount.
A really excellent non powered mount is the FlexPod, if a model exists for your car... it's very sturdy and is not obtrusive when not in use.
If notifications are an issue you can usually set the phone to something like Do Not Disturb mode. But I've never had that really cause issues.
Now about the Garmin device giving great directions - that may be true, dedicated devices have had a while to build good nav systems. I personally use Apple Maps and find the navigation for that works pretty well, it also gives land guidance... (it was better than Google Maps for navigation from Day One).
But, there also exists a Garmin dedicated app with offline maps. You get all of the Garmin benefits you detailed, only it's easier to update,
And you can switch to Waze when not needing navigation (I totally agree with you about Waze navigation not working very well). As you say, nothing beats the Waze Police/Hazard alerts.