I inserted a page and laid it out how I want. Now I’d like to reuse that same page setup for other layouts. I don’t see any way to copy page elements, or convert a page to a master page. Is my only option to re-create the page setup in a new master page? Or is there some way I can copy / convert the work I’ve already done in the page override?
I’m afraid you’ll have to recreate it. That’s the intention of the master page: if you’ll be doing the same layout design more than once, make it a master page and apply where needed.
Okay thanks! Yeah this is my first foray into engraving in Dorico… it was quite a lot of fun to tweak it exactly how I like, and it looks great, and was so easy.
You cannot save a masterpage and use it in a new document (yet), but what works flawlessly now is to save the document which has your favorite masterpage, and start all your new documents from that project. They will all share the same design and global options 
You can create a new Master Page based on your page manually:
Go to your inserted page and open it in Engrave mode.
Select your frames and make notes of their sizes (all the values you will find in the bottom property panel).
Also, if you have text frames, double click the text or tokens and make note of the text size, position and font).
Once you have all values written down, start creating a new Master Page. Here you can now make a 1:1 manual copy of your favourite page and save it to be used as a personalised master.
Hey that is a very good suggestion! Thank you 
I suggest you try this on a copy of your original file (for safety’s sake).
You would presumably base your new Master Page on “None” so it does not interfere with existing Master Pages.