As you probably know, the (GFR) or glomerular filtration rate is the test that doctors use to show how well kidneys function. They enable a standard and accepted measure to evaluate the extent of the disease, place it in one of five classifications and track its progress over time, so if yours is stage 2 then as you say it is a mild problem.
Water-only-fasting is contraindicated in severe kidney disease, and where this is towards stage 4 or 5.
Fasting will make the kidneys work harder in the elimination process, but will simultaneously heal the kidney as well.
In my view you are better to adopt a protein-restricted diet for a while as a chronic excess of dietary protein almost certainly taxes the kidneys and leads to gradual degeneration; the restriction of protein and in particular animal-proteins will aid in the kidneys self-recovery over time.