Headphone volume too loud

The lowest sound volume is way too loud on my wireless headphones (paired via bluetooth with iPhone SE). The lowest setting at <10% is the maximum I would use in a noisy environment. In a silent room I cannot use my 400EUR headset for it feels like I am standing right next to a full-blown speaker system.

I have tried all suggestions mentioned on this forum, which helped slightly but not even close to what I consider a 'normal' sound level. There are no issues when I connect other devices (digital piano, android phone) via bluetooth or use the cable with my computer. Please come with a software update to solve this issue.