This is something I tried and tested over last 3 days and has shown reduction on heating issue on my S20+.
Instead of using charger provided in the box I am using last year's fast charger that came with Note 9 to charge this device. Installed Accubattery and limited charging to 90%. Do not use the phone while charging, so overnight preferred. Start using the phone after it has cooled down from charging process.
This has also helped control battery temp overall. Earlier when external temp was 21 degrees phone would be at 42 and so. Since last 3 days I see it at around 34.
Now I don't experience heating of display and back during reading or Web browsing. Camera still gets hot when you click 3/4 picture.
Like I said this is a minor fix but hopefully helps you.
