Sounds like a case of searching for something “good enough.” haha.
First thought: Exposure Bias in the PP volume does work on mobile, so you can tune that for changing your brightness on the fly (assuming MobileHDR, I think).
Depending on number of materials, you can add a Material Parameter Collection that has a variable for night time “amount” or have some sort of custom material function. If you have this in all your materials, you can access a universal hue shift that could turn every material slightly blue for the night time. This is not good for PBR values, but I don’t know the style of your game.
If you are going for a very flat stylized look, it is possible to replicate a basic directional light that moves and changes intensity within the material. I haven’t done this, but you can find it all over the forums.
Sky materials are easy to interpolate between daytime texture and nighttime texture.
Sounds like you have a challenge ahead of you. Hopefully some of these tips are inspirational.