Have a look at sequencer mode No Gate
"No Gate: In this mode, the envelopes respond to the keyboard/received
MIDI notes, and not by each advancing sequencer step. This mode is great
for complex parameter modulation of sounds with long envelope contours,
such as pad sounds. Each new key press/received MIDI note starts the
sequence for that key/note at step one."
Sp holding keys for a chord it will trigger envelopes then, but not each sequencer step.
But not sure this goes for Arp as well. Maybe what you want can be done with gated sequencer.