This question has been posted too many times on the community.
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But there is no fix. Not until Apple engineers decide to do something. But Apple engineers do not visit the community.
My iPod nano (7th generation) has a setting called volume limit. Which when enabled, will rescale the normal volume slider. This means the minimum volume becomes lower with volume limit on. The exact same setting used to be in iOS up until iOS 11. But in iOS 12, this setting was replaced with headphone safety (a.k.a. reduce loud sounds), which only clips the max volume but does not change the minimum volume.
I recommend getting an old iPhone for your purpose. An iPhone 5 would allow you to use the volume limit feature
