If I follow you correctly, you’d like to program a sequence, but have the arp only activate if you hold a chord?

I’ve been playing around with the SID arp settings and an external keyboard recently. If you only program in single notes in the sequencer with the SID, it will only play those notes, but any time a chord is triggered (via the sequencer or a keyboard), it will trigger the arp. So you can program/play single notes, but have the arp trigger on a chord and stop when the next single note is played. So let’s say you want your sequence to play as is, but if a chord (well, two or more notes) is triggered it will start the arp. Set the arp length to max and deactivate all but the first step in the arp, and set the time to something fast, like 1x or 2x so it keeps the same tempo (obviously, you’ll need to play around with this depending on your sequence). The arp will retrigger every time a new note is played as notes are triggered in the sequencer, but will not arpeggiate unless more than one note is triggered, and will continue playing the arp patterns until the next single note is triggered. So you can do something like play an alternating two note bassline, but play a chord on the third bar to trigger the arp for some variation, then return to the pattern on the 4th bar by returning to single notes. The arp will continue to play until interrupted by either a new chord or single notes, so while it may not be totally what you have in mind, you may be able to find a workaround by chaining different patterns.

No matter what, I don’t think it is possible for any of the arp settings to pick up in the middle of the sequence without triggering the arp from the beginning, so you can’t just start it from the 8th step in the arp. Not sure if this is what you are asking, however.