I dont know if they fit with your hearing aids, but maybe look into bone conducting headphones. I have Openrun Shokz Pro and they work pretty good with music. They have a waterproof version which is good for your hygiene needs.

Your Bluetooth speaker around the neck idea might be hard to achieve. The wavelength of a 100hz signal is around 3,5 meters and is getting bigger the lower you get. This means you need more energy and bigger drivers for true reproduction of low frequencies. A 100hz speaker might get heavy around the neck.

Sidenote: When I'm wearing my Shokz I sometimes get asked if I wear hearing aids.