Hello, I've crappy TVBox (H96 Max+ RK3328, 4GB RAM, 64GB Storage) It has Android 9 installed on it but I want to install AndroidTV is it possible, if yes where can I find the correct ROM for it? If it's not possible is there a way to install Leanback Launcher into this device (Since it has a phone/tablet version of android Leanback is not supported) and install TV versions of the Apps?
You might look into the ATVXperience rom. Not sure if it's available for that device/chipset, but it can get you something closer to the Android TV experience.
To be quite frank about it, though, it might just be better for you to trash it and get yourself an official Android TV device. There are some really budget friendly devices on the market now.