Sarah We solved this using Draft Orders + Stock Reservation. We did a video on it here - https://youtu.be/mRUcIfXICOw
Basically what you do is create a Draft Order on Shopify from the Shopify Admin, add all of the items you want to reserve to the draft then reserve the stock. You can set a particular day to release it back, or set the date for a really long time. Once you’ve saved that order, to check its working have a look at the Inventory tab and see if its marked as unavailable due to the draft order you created.
Hope that helps, thanks, Andy