Hey Tim,
This is John from the Shopify Guru Team. :)
Check out the app, Bouncer.
Your SEO won't be affected by this. Google allows modal overlays that open within the same window that a visitor is browsing. Google does not however, allow pop ups that open in a separate window outside the window the visitor is browsing, which isn't the case here.
I hope this helps. If you have any other questions, don't hesitate to reach out. :)
J