You do not have to buy airpods.
Any microphone / headphone combination will work.
In fact a directional microphone paired to a reference quality headphone would work very well. No need to use a phone in the middle either, many options for little dedicated amplifiers that would be easier to use.
Usually hearing aids are tuned to an individual user (and individual ear)'s hearing response frequency curve, kinda like an equalizer, that adjusts volumes differently across their range of hearing. To do that their hearing loss must be measured first and saved into a profile. There are already apps that do that, and you can simulate it yourself with the right methodology and equipment. But a dumb amplifier that just makes everything louder won't do that. It might make some frequencies too loud for comfort while insufficiently amplifying the ones you need.
Using standard equipment (iphones and airpods) makes the calibration easier, but it still needs to run an individual profile.
In fact a directional microphone paired to a reference quality headphone would work very well. No need to use a phone in the middle either, many options for little dedicated amplifiers that would be easier to use.