I know there have been some changes on the G5 so I'm not clear how that works.
With the G4 the Dexcom CGM acts as a proxy to the phone. So when everything is working correctly I get two alarms. So this gives me a certain level of redundancy. Anyway it works for me well enough that I can get a full night's sleep. For years I had alarms set to wake me up several times during the night.
[EDIT]
Also, responding to your question about the phone being louder than the Dexcom. The phone is an order of magnitude louder than the G4.
Also though, which model do you have?
I have the G4 Platinum w/Share. The sensors for it now ship with a warning sheet stating that you may not receive audible alarms due to issues with the speakers.
I have G5 with Share (I think; I have a drawer full of old Dexcom stuff) and have gotten the same letter, but haven't had that issue. I definitely trust it more than Dexcom pairing with the phone. I'm glad your setup is working for you, though.