Different phones really do have different volume characteristics.
My cheapest phone actually has boh the best volume and sonic quality- go figure.
Generally, the apps you are talking about will not help with in-call volume, as they are designed for music, speech, and media use instead.
Android only allows one sonic modification app to run at a time, and these apps are shut off during phone calls.
As for your question about ring and notification volumes being linked, this is very common with Android phones.
As
@Hadron mentioned, there are multiple different volumes on an Android, and there are even different volumes for a phone call.
The earpiece and the speakerphone have separate volumes.
Bluetooth devices and plug in items (to the headphone jack) will also have separate volumes.
All of these can (and should) have separate volumes from each other.
The trick is that you can only adjust a specific volume while you are using that one.
So, if you want to adjust the earpiece, have the earpiece on while making the adjustment. The same goes for all the rest.
If you still have a hard time hearing during a call, I would suggest to try using a set of headphones if possible.
A set of $10 headphones can go a long way to making the sound of a call much more pleasing.
Here is an app that you may find useful.
At least it will give you quick access to most of the volumes on your device, and it can also show you if indeed your notification and ringer volumes are linked.
It can also schedule volumes for you if you like.
Sound Manager (Volume level scheduler) -
https://f-droid.org/packages/com.roozen.SoundManagerv2
