Hello: With copper running directly into the tank, there is likely rust building up at the inlet and outlet. I’d redo them both along with a new relief valve. You might consider flushing parts (sold elsewhere on this site) rather than a chemical delimer. It makes the task of flushing in the future easy.
If you get poor flow from the drain valve with the water left on, the inlet is likely clogged. Otherwise it’s the outlet of the heater. Since you’re going to redo both, that tidbit may not be too useful to you… perhaps to others ;).