Szuwi
With the apps Keymapper + Easy Sound Recorder, I can place the recording function (start recording Immediately) under the volume button. In other words, I can recording directly sound when I press the volume button without having to unlock the phone first.
you only need to wake up the screen first by touching it.
Unlocking is not needed (no my phone is not rooted)
You mention activating Fossify record with volume button, but then you only start the app, not the record. You still need unlocking the phone to start the recording. With Easy Voice Recorder, I don't need to do that.
You said, 'I don’t think it’s a good idea to use an app like KeyMapper. It has access to a lot of things...'
Well, if you disable the permission connect to the internet, then who cares? It can't communicate outside your phone anymore, so it can't do any harm.
i also do this with Easy sound recorder.