The developers options 'stay awake' should be fool-proof.
As far as I know, it is the most reliable way to keep the screen on.
If it is 'hit or miss', then check the charging cord that you are using.
Also, keep in mind that this setting will not stop the screen from dimming, only from shutting off.
So, your screen will dim instead of sleep after the sleep time you have set in display settings.
There are a few apps available, but as I said- 'stay awake' in developers options is the most reliable. For it to work, the phone must be constantly charging.
An option you may not have considered is an app that wakes the device whenever the proximity sensor is covered, then uncovered- such as waving your hand slowly over the device.
Two benefits are that you do not need to constantly charge the device, and it saves a great amount of battery because the screen is only active when you want it to be.
I use an app for this quite extensively, called Wave UP.
Here are some links for some apps to try, Wave Up being the most recommended by me.
https://f-droid.org/en/packages/com.jarsilio.android.waveup/
https://f-droid.org/en/packages/com.github.muellerma.coffee/
https://f-droid.org/en/packages/info.zwanenburg.caffeinetile/