The ideal setting would be:
Strength: 2
Maximum gain: 2x to 3x.
Output threshold: -1.9dB (or more if it starts to distort).

Warning
Continued listening to the audio on higher volumes might damage your ears and your equipment. Please be cautious.
Master power
This option must be enabled for the mod and settings to work. But when you expand the menu, there are some settings such as Output gain, pan, and threshold limit. Here leave those settings as it is, and only change Output gain values. This reduces distortion as this modifies the final processed output.

Ideal settings are:
Output gain: -1dB to -3dB for removing distortion. +1 to+3dB for more volume boost.
Threshold limit: 0dB
Dolby Digital Plus
This mod specializes in enhancing the audio experience in music or movies. But there are specific settings that can boost headphone volume. Let’s see them now.
Dolby Digital Plus Magisk Module
Equalizer

Equalizer gives us the ability to tune certain bands within the sound to increase its dominance over others. For example, you can set bass (low band) to dominate over treble (high band).
But in this case, we are going to increase all bands by a small step. This increases all bands and results in a volume boost. You can also play around and obtain the best suiting settings that match your need.
Volume Leveller

This option will level up the volume and will result in a volume boost. This option does not have any fancy tweaks. Its just a toggle button. So simply tap on it to enable.
Support libraries
So, now that you are using mods to enhance the audio output of your android device, there is a need to install some additional Magisk modules for smoother operation. But don’t worry, they are available within the Magisk repo, and you can install without any hassle.
Audio Modification Library

This module allows seamless working between multiple audio mods. Especially when you are planning to work between multiple mods to compare and choose your ideal one.
Audio compatibility patch

This module is required when playing audio from streaming services like Spotify and Youtube.
Conclusion
So, to sum up, these were the Magisk modules that you can use to install to boost headphone volume on your one plus device. But my choice would be viperFX since it has better control options to play around with. But for novice users, Dolby digital plus would really help.
The sweet spot is you can use both of them, but remember to apply only one mod at time and switch according to your needs. Just don’t over vamp any settings.