Flow to add subscribers after they finish an automation

I am creating an automation campaign as a welcome series for first-time subscribers that have about 4 emails in it. As a best practice, you should avoid sending other emails (such as newsletter etc) until they finish the welcome series automation just to avoid over-sending. Because Shopify Email won’t let me say what segments I want and don’t want to send to, is there a flow that would let me create a segment that puts the subscribers who are in the welcome series automation and after X amount of days adds them to my “all subscribers” segment?

I hope this make sense! Thank you!

Can you tag them and then check the tag before sending?