I have to keep this digital piano as it's past the return date but apart from changing the key sensitivity, is there anything else I can do?
Not really. At least from the keyboard actions I've taken out of digital pianos, there's slight weight added physically to each key that you might be able to remove or trim down.
That would be for each individual key, after fully disassembling and removing the action. That might take half a day or so of pretty tedious work, if it's even possible on the Roland mechanism (most keyboard actions come from one or a small handful of manufacturers, regardless of the brand of keyboard, though). And that's from memory a long time ago.
Your other option you already came across: adjust the velocity parameters such that you don't have to depress the key fully to create tones.
AFAIK, those are the two options.
/* EDIT: well, there's a third option, which is to buy a new keyboard! I used to have a Kurzweil SP-76 (I think it was called), which had a "semi-weighted" action. It wasn't like a synth keyboard, i.e., pretty much unweighted, but it was close to a spinet action. It used some kind of springs in the action, but I never opened it up to look. Very light, very good sounds for the time, and had lots of professional features like advanced MIDI capabilities. It was actually pretty easy to play something resembling full-on classical music on, despite it having been a very light action. Something like that, probably.
OR, you could buy very cheaply an unweighted MIDI controller (that is, something with no sounds, just a slab of unweighted keys and a few buttons to control various things), plug it in to use the sounds from your Roland, and maybe gradually over time start switching over to the weighted keys. Any old keyboard that has MIDI output should work just fine, even a child's toy, something you'd find very cheaply indeed. */