Perhaps this isn’t something that you want to hear, but I’d like to point out that the game will be at least 1.2 to 2.4 TB in size if you are going to end up using that many textures.
It is not feasible to create a game with 1,000,000 textures, each in the size of 1024x1024.
The large number of textures, each with a high resolution, will take up a tremendous amount of memory and storage space. This will significantly impact the game’s performance, as loading and rendering so many textures will put a strain on the GPU and CPU. Additionally, it will take a long time to load all the textures into memory and it will also be a big problem to store all the textures in disk.
You will need to download textures from a server as you go and play the game, and you would have to setup an infrastructure to carry the capacity of all users playing.
If you wish to convert textures into sprites, there is a bulk action that you can run to convert the textures, alternatively you could build a script to do so using C++ or the Blueprint Widget functionality.