The battery health dropped from 98% to 95% since I switched to 18W in April. So,anyone need a 18W Anker usb-c charger or cord lol.
To jest, I wonder what might have happened had you used the new 96W USB-C bricks that come with the 16" notebooks to charge the phone using a USB-C to lighting cable.
On a serious note, the only real issue today is heat generation that dims the light on battery life, not much else over regular use. Playing games that kill the battery soon while producing heat and then charging the device using a fast charger would be a somewhat extreme example of how to reduce battery life soon. From what it looks, the decrease in your phone's battery life is contained and normal, it is just coincidental with your using an 18W charger. Enjoy your phone.
I use a 29W USB-C charger with USB-C to lightning cable. What I don't do is let the battery go down below 60% if I can help it organically. I don't fret, but I just put it to charge when I can, and don't bother when I am out. I realise that using a fast charger on a battery that is low on charge at, say, 40%, will generate more heat than using the same charger on a battery at, say, 60%, since the charger will pump a higher voltage into the battery to charge it fast. Yet, at somewhat reasonable charge levels, say 50-60%, the fast charger is fast enough and cool enough. So, I use fast chargers knowing I can charge the phone
really fast if needed when the charge is low, and to just charge the phone
quickly enough when the charge is adequate.