You want to flush out the bottom few gallons maybe annually. This can be done while the tank is under pressure. You still might want to combine this with your T&P changing.
The amount you need to drain is going to be more if the T&P is on the side. Mine have been on top, and were gas WHs.
I was not able to use one of those plastic drain tubes. Leaked. I ended up reusing the galvanized drain pipe, even tho with the taller T&P I chose on purpose, the bottom of my pipe ended up a little higher than the required 6 inch max above the basement floor. The good use of galvanized in plumbing IMO is for T&P drains.
I needed to use a lot of torque to take the galvanized apart. I used a large curved jaw locking pliers and a 24 inch pipe wrench. I used both PTFE tape and pipe dope. I also had a large adjustable wrench that I used to tighten the T+P I think, but I did not take notes. I may have forgotten details. You will need to tighten to a position that points the output of the T&P the right way. It does not have to be very tight, but I know that is ambiguous.
ONE MORE THING: if leaking is intermittent, then it is likely due to thermal expansion, and that would mean that you should have a working thermal expansion tank. When water heats in a closed system, and the pressure gets to 150 PSI, it is normal for the working T&P to release water. Cities are adding check valves along with water meters, so as time goes on, more people will need one of those.