I think Lissandra should be able to pop her thralls early

I'd actually think that would make her a lot more interesting than just loading it up with an absurd amount of base damage if she had some degree of control over them herself - namely, being able to detonate them manually.

Something like being able to basic attack them, or use her Q to either reduce the timer a little bit or pop the ghost altogether -- in exchange for some of the damage, would make her a lot more interesting for both her and her opponents to play around the thralls and allow for creative play between all parties involved.

If you wanted to give her a choice to hold off on it and let it pop itself at the 4 second mark, you could have the damage vary based on how long it's charging, which lets Lissandra measure reliability versus reward.