Buy headphones that not only allow you to turn down the volume, but also to turn it up as much as possible. - Such headphones usually cost a little more.
(An alternative is to watch TV with the iPad. The iPad may provide a stronger sound).

Perhaps you should check whether the loudspeaker output on the TV needs to be activated or whether a setting needs to be made so that the sound is primarily output via headphones?

Unfortunately, I don't know the AirPods, but maybe they save the last volume setting (from the smartphone) and would have to be "turned up"?

Sorry, I can't think of anything better.
Good luck!