Yes, you should be able to do that, but it depends on the Z-Wave parameters. There are 4 Z-Wave parameters that control dimming:
- Dimming Speed
- Dimming Speed (Z-Wave)
- Ramp Rate
- Ramp Rate (Z-Wave)
Dimming speed is how fast the switch increases or decreases power level, and ramp rate is how fast the switch moves to/from “off” state to the desired power level when the switch is turned on or off.
If you want “instant on” + Dimming, set the Ramp Rate to 0 (so that it jumps to/from the target power level instantly when turned on/off), and set the Dimming Speed to your desired dim rate when holding the switch up or down (3 is the default, higher is slower).
As indicated, you can also set separate rates/speeds depending on whether someone is physically interacting with the switch or if a Z-Wave command is triggering the dimming/state change. By default, parameters 2-4 are set to 101 which causes them to mirror whatever parameter 1 is set to.
I’m not particularly familiar with home assistant, so I can’t provide exact instructions there, but the switch should do what you want.
LZW31 Z-Wave Parameter Documentation