Considering your requirements and that you do not wish to pay for a photo-editing editing programme, I would recommend Photopea for designing your new book cover. Photopea is free and it has tools, interface and features very similar to Photoshop.
What you should keep in mind is that you still have to spend some time practising to learn how to use this programme. However, if you are familiar with Photoshop and already got graphic design skills, it is super easy to familiarise yourself with the user interface of Photopea (since it is the same as photoshop).
To get to know the basics fast, I suggest you to read some tutorials for beginners and watch videos on Photopea.
I hope that you will manage and kudos to you for deciding to design the book cover yourself - If you need anything feel free to reach out 