I have never trusted my cell phone as an alarm due to my anxiety of if I set the volume correctly or not. I'm rocking my Sony Dream Machine that's got to be over 20 years old now. Works perfectly, would recommend.
I understand your concern, but every app should now have volume override. My native Android alarm and Alarm Clock Extreme has worked flawlessly for years despite my phone always on vibrate. Extreme makes me do math or a captcha before dismissing