Hello: Let’s try this in order…
Rheem anodes have a resistor built into them. There is a grey plastic ring under the hex head, above the sacrificial metal. Magnesium is springy and difficult to bend. Aluminum is soft, easy to bend.
Teflon tape is good.
You can add a second anode, but it should match the original anode in metal type. Understand that if one anode has a resistor and the other rod doesn’t, they will not be used up at the same rate. Heater ports vary. Some are easier to add an anode to than others. Sometimes there is a steel ring in the way. This can be drilled out or the plastic of the anode top filed down to fit.
Unlike sodium, magnesium is stable in air or water 😀 Go ahead and get spares.
Replace the plastic drain with these parts, in order, from the heater. Plastic lined steel nipple — brass ball valve — brass hose adaptor. This arrangement puts distance between the steel tank and brass parts, mitigating the corrosion that happens between different metals.
A filter upstream is unlikely to have any effect unless you’re on well water.