Always doing fast charge while turn off the iphone, is it safe ?

Hello , i was thinking about fast charging health issue ony iphone xs . when my phone doing fast charge with my original 20w apple charger, my iphone got extremely heat temperature around 41 celcius degree even with airplane mode on and i not using it while charge. That charger ia my only original charger that i had ( i still dont hava a plan to buy 5w charger coz the price still too high). I want to charge my iphone without had heating issue like this. So i realize that the only one thing that i can do is charging while my iphone turned off. I charge from 18% , turn off my iphone & wait around 20 minutes and test the temperature its completely normal around 36 celcius degree constantly. After 2 hours charging (with 20w adapter), the battery reach 89%. I think that was a little bit slower than when i charged in while turn my iphone on but still bettee speed than using 5w charger. But most importantly the heat issues is gone. I want to make this to my daily charge habit (maybe i will charge my iphone 1-3 times daily depends on how much times i using it). But i want to make sure is it really safe to do so ? Will my iphone and battery will be damaged faster with that habit ? Thank you