I am experiencing the exact same problem and for the same reasons. I dislike how the content being actively worked on for my site is live the whole time. Just looks sloppy on my part even though there isn't anything I can do about it other than consider alternative templates or platforms.
Another issue I have that is related to this problem is there doesn't appear to be a way to password protect individual pages within the portfolio folder either. This would be useful so that I could lock down content for specific pages that might have projects under NDA (among other reasons).
I've run into a bit of an issue with hiding individual collection pages in Squarespace 7.1. Normally, by clicking on the settings for a page, you are able to enable or disable that page. However with the new system of "Portfolio Collections" that page disable option does not exist for individual portfolio project pages. I may be mistaken, but as far as I can tell, there is no way to hide these pages (screenshots attached for reference).
This is incredibly troubling as you cannot create drafts of projects, nor if you are duplicating other projects, they will be live with duplicate content until you fix it.
I was thinking possibly that you could move collection pages to other collection pages(in the way you can move products from shop to shop), so in essence you could pre-build the portfolio page on a hidden portfolio collection and move it to the live portfolio collection, but there doesn't seem to be a way to do this either.
Any advice on how I could make hidden pages in the existing Portfolio Collection system or is this just a fault in the new version?