I am unsure of what specific options are available to you because you are using a discontinued device that is unable to update to the current Android/One UI version, but you could try using Modes & Routines to create a routine to extend the screen timeout during a call.
If you are running OneUI8.0 and have Good Lock installed, download the Display assistant module. If you don't have Good Lock installed, what are you waiting for, download both. You can set the phone to trigger the screen to stay on for 10 minutes (I've heard this may not work on every phone model, but that was when it was first introduced) and you can add a toggle to the quick panel to keep your screen on for 30 minutes. Both options work perfectly for me. The 30 minute screen on is especially useful in other apps besides the phone. I don't understand why you can't answer people's questions while the screen is dark, unless you mean you have to press the keypad to navigate the voice menu.
