Hi Daniel, I'd go ahead and get everything ready now. When you first make a bed with bagged 'soils' or other things it takes awhile for the microlife to totally inoculate the bed and getting worms. Also, if you have them built and ready this summer, you can start to improve the soil in the beds by adding wood chips inoculated with mushrooms or growing growing cover crops.
You would also have the option of building a cover on one to extend your growing season this fall or next spring.
A piece of land is worth as much as the person farming it.
-Le Livre du Colon, 1902
I would fill the beds now. This way everything can settle (and you will see that all the material will compact quite a bit).
And you never know what will happen next year, life can get in the way of your plans and then you will be glad that you have those beds waiting for you. But make sure to mulch them / cover them up in a way so that you don't have a weed problem next spring.
Or you can throw some seeds on top which don't need your care (sunflowers, green manure, pollinator mix...).
I am only one, but still I am one. I cannot do everything, but still I can do something; and because I cannot do everything, I will not refuse to do something that I can do. (E.E.Hale)
I also would fill the beds now. You could plant a cover crop or simply mulch it to add more organic matter. Not to mention that when the urge to plant arises, your beds will be ready.
I've built seven new beds this year and have used mostly found materials for my soil though I did mix in what topsoil I had on hand as well. While it has been a learning experience on what works and what doesn't, it would have been much more beneficial if I would have constructed the beds last fall as the soil would have had a chance for the worms to populate and mix in the organic matter a bit.
Fill them now and either cover them with something to block light or get some seeds and just throw them in, pick something that will die over winter and not seed before then and you'll have a good headstart next year.