Another fun source of background images is Astronomy Picture of the Day by NASA. About half of the featured pics are released by NASA into the public domain.
https://apod.nasa.gov/apod/ap190421.html
Also, BlamBot is a wonderful source of dynamic fonts which are well suited for covers. A ton of them are free ti download. Blambot allows self-publishing and small publishers to use these fonts for free, and large publishers can negotiate to use them.
By the way, I really like the composition of your sample covers, Poppyqueen. I see a lot of good use of color, contrast, and arrangment there.