This topic is covered quite often. There seems to be some consensus on this forum that a diet consisting only of mice is fine and variety is not always such a great thing.
To be honest, I have only raised up one water monitor from a hatchling. That was enough for me!
I fed mice, birds & roaches when she was young. I can’t recall offhand when I stopped feeding roaches. She stopped showing interest, so I stopped offering them. Then I fed only mice after running out of birds. And she does fine on mice as an adult.
I really do think that for a young water monitor, a diet consisting only of mice will work well.