The Quickstarter is deprecated, as of libreoffice 5.4. From what I can gather, it was finally removed in version 6.1. I was sad to see it go as well. Like you say, it sped things up quite a bit, especially on slower hardware. I guess I should really be upgrading some of my 1990's hardware... :)
In all seriousness, I think the answer is basically no, you can't use the quickstarter, unless you want to design some kind of custom script that would run the soffice app in the background somehow without opening a window, while still allowing libreoffice windows to open... Alternatively, you can simply put
libreoffice in your startup applications and simply leave the basic window open on different workspace. Yeah. Annoying, but I'm afraid that's the best you'll get.