I've had the same issue with some wired IEMs on iOS, using Apple's official USB-C to 3.5mm adaptor. I even have the EU model A2155 of the USB-C to 3.5mm adaptor that's supposed to have half the power of the US model.
What I found that helped was to create a custom Shortcut that "Set Media volume to 1%". iOS reports that this is 48 dB when playing pink noise. I managed to hit 47 dB when dragging to volume slider on iOS below 1%, but the Shortcuts app only seems to support integer percentages.
In my case, even the 1% volume level was too high in a quiet room, but some apps have a custom EQ setting that you can use to lower the volume further. E.g. if you're using Apple Music, you can go to Settings -> Music -> EQ and pick "Loudness" to lower the volume further.
Unfortunately, they're not great to use with a phone in your pocket, since then the motion of walking around will adjust the knob and change the volume.
Although, come to think of it, I could probably just glue the knob into a fixed −10 dB of attenuation and then use software volume control to change the volume.
I have also noticed that those passive inline volume knobs tend to adjust the right and left channels by different levels, especially with low-impedance outputs like IEMs, but that might because the ones I've bought cost ~US$2 from AliExpress.
I could probably also fix the issue by buying a worse/less-powerful USB-C to 3.5mm DAC. The official Apple one is pretty well liked by the audiophile community, since it's powerful for the price, which is great if you have high-end headphones, but horrible if you have earbuds/IEMs.
Weirdly enough, the same Apple USB-C to 3.5mm DAC is much quieter on Android, since it defaults to a low hardware volume on the DAC, and Android then only uses software volume control to lower the volume, see https://issuetracker.google.com/issues/242221770.
What I found that helped was to create a custom Shortcut that "Set Media volume to 1%". iOS reports that this is 48 dB when playing pink noise. I managed to hit 47 dB when dragging to volume slider on iOS below 1%, but the Shortcuts app only seems to support integer percentages.
In my case, even the 1% volume level was too high in a quiet room, but some apps have a custom EQ setting that you can use to lower the volume further. E.g. if you're using Apple Music, you can go to Settings -> Music -> EQ and pick "Loudness" to lower the volume further.