M-Files Vault List

You can use it with larger lists, as a general case.  There will be an initial calculation (not immediate for 1,800 items, but it shouldn't take too long), and then the calculated value will only be re-calculated when the object is modified.  Note that it will make a change to the object when it does so.

I'm not sure that duplicating data to another list makes much sense.

I know I've asked you in the past whether you work for an end-user or a reseller and you've not told me, which is fine.  If you work for an end-user then I would suggest you reach out to your partner for some assistance; they'll be able to take a closer look at what you have and what you want to achieve, and guide you on some of these built-in features.  If you work for a partner then open an Implementation Support Request and get guidance from our dedicated partner-support teams.

I remember saying it, but let me say it again. I am an end user.

Apologies - maybe I missed it.

I would recommend speaking to your partner about these questions.  We will always try to help on this forum, but it's often difficult to come up with a perfect solution when you can only see one part of the puzzle.