RH and LH dynamics

You can select one RH note, right-click, select, all similar in same staff, right-click one of them, note properties, velocity type to User or Offset and then increase velocity absolute resp. in %.

Or you can create a piano staff from 2 pianos, one with only RH and the other with only LH and then use dynamics independently on either.

Just a note if you are new to writing for the piano: if you have a series of chords in the LH against a single note melody in the RH, that's likely to not balance well in real life either. Real pianists would do their best to bring out the melody, but notes on piano decay quickly, so if the melody is longer notes while there are lots of chords underneath, the chords *will* drown out the melody. That's the nature of the instrument. So this could be a good clue that maybe you should find another way of arranging this passage.

If on the other hand you are experienced at writing for the piano, then of course you know this - I don't mean to insult you here. I just can't tell from your message if you aren't aware of this, or if you know that this happens in general but feel that your particular passage would still balance better than it does - no one would ever claim computer playback captures the nuances of a real instrument perfectly. So there *are* things that work in real life better than in simulation - and vice versa.

You should be able to select a dynamic and in the inspector (in a nightly, press F8), you can choose if the dynamic apply to a part (the default); the full system or only the staff (what you want here).