IEMs are usually low impedance but you will have better luck if you dont mind the earbuds.
Another option is a impedance adapter.
you can get 50ohm adapter and all your IEMs can activate the volume boost.
HiFi DAC can be enabled regardless of the IEM Impedance.
The volume boost is your question.
Another trick is... cumbersome though....
you can use an adapter(not impedance adapter) and then plug in the impedance adapter and your iem.
V30 will detect the high impedance and activate volume boost.
then remove the impedance adapter.
and connect the IEM to the reegular adapterand the volume boost stays on.
LG V30 - normal adapter - impedance adapter - IEM
LG V30 - normal adapter - IEM
What this does is remove the influence of ohm adapter.
You dont really need to do this.
Turning on the hifi dac is usually good enough.
Are you using planar IEMs?
That also wouldnt need volume boost is what I think because it is the voltage that increase.... not as much the ampere.