hey @christopher m​ several solutions:

1. Series as you mentioned - since this is on 6 months I'm pretty sure you can build it out and have 1 (after 2 months) + 6 surveys (each 6 months) in line as that will cover 3 years. Businesses will definitely change the content of the NPS in the 3 years, so you will anyway have to create something new...
2. Use an individual survey with event-based triggering - your outside logic should, for each user independently, fire out event, let's call it SurveyTrigger. And then just select that event as a triggering event. I'm not sure what you mean by "And the event trigger only allows for last survey date without letting you select a specific survey" - as you are mentioning a date, I'm assuming you wanted to use the date trigger in the When to send section - be careful with that, I remember that I had a long discussion with Intercom about it as it did not make sense to me back then, and it turned out that this can not work with automatically set date attributes like Signed up, Last seen (as obviously, those dates will be in the past, not the future, so you can not use "before the date" but it might work with "after the date" and "on the date" - I haven't tested it).

Thanks Milan - these mimic the conclusions we came up with internally. I agree with 1 as it will probably change bit it would be nice to be future proofed. We utilize a version of number 2 for our enterprise customers in another system, so looking at ways to mirror that in Intercom for the subset of customers (tech touch) that my team works with.