Funnily enough when we bought this house seven years ago I got a bunch of acetic acid to do a thorough flush, but I couldn’t get the anode out. Still can’t get the anode out
All of that white nastiness is limescale, we have some of the hardest water in the country. The local saying is that it takes one guy to carry a water heater into a house and three to carry it out!
I’ll flush it again before we leave in the summer (renting the house out), though I’m not sure how much scale that will remove without any chemical help