I'd like to correct you on the lab blocks. A quality lab block such as Harlan or Mazuri provides a rat with everything they need. No supplements needed. And quality blocks won't promote tumors. In fact many members here have seen tumors disappear due to feeding a quality lab block after feeding a poor diet. (feeding rats in the colony a bad diet vs a good diet. When feeding a poor diet, the rats developed tumors. When switched back to a good block, no tumors arose)
However, feeding a poor quality lab block is the same as feeding garbage...
Even fancy rat breeders will state that a quality rodent block is fine alone. Many rat clubs sell Harlan and Mazuri as a main diet item.
Blocks can also be better than a mixed diet as the rat doesn't have the ability to pick and choose what it eats. In a mixed diet, they would have to eat everything in the mix to get all the nutrients needed, and some rats don't do that. They pick what they like and leave the rest.
If you give too much fruit, they get diarrhea. Too much citrus can promote kidney tumors in males too. At best, I would supplement fruits and veggies as they're mainly a grainivore.
I breed and raise rats as feeders and pets. My rats live in my bedroom and live a pet quality life. I give my rats fruits as treats. But overall, they eat Mazuri lab block.
They are healthy and look as good as show rats.
And it does matter whether or not a feeder is put on a good diet. What goes into the feeder goes into the snake. I want to feed my snakes a good quality rat so I feed my rats a good quality diet. And that consists of rodent block.
I also work in a research lab. We feed rodent block to the rats and mice as well. And as a a scientific model, the animals need to be healthy. The block provides a fully balanced nutritious diet in order for them to thrive.