These are both great things you have learned. When I started using serato via my twitch controller, I too was “mixing with my eyes” and it was always a couple of beats off, requiring more manual correction (I don’t use the sync function). Mixing with your ears, at least when dropping in the new track, is the way to go. For very tiny corrections, I will reference the waveform display, but that’s it.
As far as using the volume on the channels for mixing (with the fader left in the center), I am torn about this. I’d like to try it, but what I find stopping me is that I often adjust the bass on the incoming and/or outgoing track, thus nothing having enough hands to control two volume levels and at least one bass EQ knob. So, I stick to mixing with the fader. Works for me. But if volume mixing works for you, then more power to you!