I'm starting to wonder why DPs even allow to change touch sensitivity because you can already adjust volume level if it sounds too quiet or too loud.
There are two different things involved, therefore there are two ways to control these two different things:
The first thing is the actual volume, which you control with the (you guessed it) volume knob/slider. A good baseline for this is the following:
Try to get some experience playing on an acoustic piano (different ones, preferably - maybe in a shop or so) and get a feeling for how loud the piano is when you play with low force, medium force and strong force.
No adjust your DP volume to be about the same (it doesn't have to be exact, different acoustics also have different volumes).
So now you have a DP that sounds about as "loud" as an acoustic when you play with low/medium/strong force.
That was step 1.
However, there is now also the quality of the tone, the timbre.
If you check this on an acoustic, you will notice that (unless the acoustic is totally clapped out), when you play pp/p the piano is not only quieter, but also more mellow than at mf. And if you play f/ff, it is not only louder, but also more harsh.
So the range from pp to ff not only changes the volume from quiet to loud, it also changes the tone from mellow to harsh.
Now compare this with your digital. A good digital these days should also have such a timbre change from pp to ff (at least somewhat). Does this sound natural to you? Does the pp/p sound suitably mellow and f/ff suitably harsh?
If yes, then you are all set.
But what if not? Then it's time to change the touch settings. If the piano does not sound mellow enough at pp but too harsh at ff, then you need to shift the touch in the direction of hard. This moves the whole dynamic range down a bit, towards the mellow pp end. Or if the piano sounds too mellow at pp and not harsh enough at ff, then you need to shift the touch in the direction of light. This moves the whole dynamic range upwards a bit, towards the harsh ff end.
That was step 2.
Now, if you actually changed the touch setting and moved the dynamic range, then of course you did not only shift the timbre, but also the volume. If you shifted towards hard, then playing with the same force as before will now produce a slightly mellower tone that is also quieter. Or if you shifted towards light, then playing with the same force as before will now produce a slightly harsher tone that is also a bit louder. To compensate for this, you should now adjust the volume a bit in the opposite direction.
That was step 3.
Ideally, you now have a digital piano that, when played with a certain force, produces about the same volume and
the same timbre as an acoustic piano would produce when played with the same force. This would not have been possible if you only had the volume control, because while you could then adjust the piano to have the same volume, the timbre may not match that.
Disclaimer: This is of course an ideal scenario. In the end, you will probably have to reach a compromise, as the combination of volume and touch adjustment likely won't allow you to achieve exactly the volume/timbre combination you are aiming for. Also, all acoustic pianos do of course behave and sound different (both regarding volume and timbre), so where I wrote "same as an acoustic piano", that does essentially mean "same as you think an acoustic piano should sound" or "same as the acoustic piano you are used to sounds".