Well, you can reference the below SL criteria! You'd need to update/modify the filters based on the nurture transitions and also what means conversion in your Marketo instance. Whether it is, Converted status on the Person Status field, RCM stage transition, etc.

But this SL criteria clearly comes with a lot of caveats and assumptions -
1. You don't have a flow to remove people from the EP(s) (most people don't remove people from the program, but worth mentioning).
2. You have transition rules to move people to a different stream when they exhaust the content, if you have the campaign to manage the exhausted people transitions, then you can reference the campaign membership criteria as well instead of filter #2 (and that'd be more accurate than using the Engagement Program Stream Changed filter, as non-exhausted people could also be added to a different stream.
3. You'd wanna use the Date of Activity in the Converted status filter to be after the EP was launched, else you'd have people in the SL that'd have converted before the EP went live.
You'd most certainly not be able to pull a very accurate result of people using SL, as you'd not have just EP driving your lead journey, EP would essentially be a part of what contributed towards the conversion but not just the sole reason. And, of course, not leaving behind the fact that the CDV activity is only preserved for 90 days from the date of activity. You'd wanna do an activity export of members with conversion status using API, and analyze if their conversion date is after they'd changed the stream (if that's really what you're looking for).
Also, once the leads received the last email in the stream, don’t the leads get removed from the nurture program stream automatically?
Nope, they're marked as exhausted leads (check the Exhausted Content column in the Members tab of the EP), Marketo wouldn't transition them to a different stream until you have a campaign flow/transition rule for it.