List, Segment or Flow?

Hi, I can't figure out what is best practice in this case: We have a

• Shopify website that needs all standard emails like abandoned cart etc
• A newsletter signup on that Shopify site, triggering a welcome-series.
• An external landing-page hosted on Unbounce that triggers a waitlist-email.
• A lead-form on Facebook that also should trigger the waitlist-email.

If I sign everyone up to the main newsletter list, then I need to exclude the landing-page and Facebook subscribers from the welcome-series (as well as any other potential future sources).

Should I then trigger the welcome-series not by joining the newsletter list, but by filling in a form?

Or should I create a new list for the waitlist that holds Facebook and landing-page subscribers and sends the waitlist-email when someone joins that list?

Or should I create a segment "waitlist" and exclude that segment from the welcome series?

Thanks for any advice

Dirk