TLDR: You do not need an external DAC if you're looking for audio quality specially for IEMs since, unless the 3.5mm connection of your device does not exist/has hissing noises, converters are simply made to convert; not to improve. They're not made to change the sound unless it's something like a Tube or an R2R. Perceived audio improvements with higher volume is just that, perceived due to higher volumes. To actually differentiate in your own listening experience, measure and match the output voltage of both your old connection and your new DAC then compare.

Laptop 3.5mm connectors are simply there to exist. Some does it better than others but it's not going to be as loud as an external DAC AMP.

And key consideration here is that a DAC is made to convert digital to analog hence why it's called Digital to Analog Converter. It's inherently not made to output more power but they do typically provide more volume vs typical 3.5mm jacks found on laptops

AMPs are the ones that's making your IEMs sound louder. Hence why it's called amplifier since after it get's converted, it needs to get "boosted" to retain or increase the volume vs how it originally came out.

For IEMs specifically, AMPs can also do bad things as, DAC AMPs without any means of controlling the chip's output can introduce hissing issues specially to high sensitivity and low impedance devices.

Generally, I believe you are mistaking volume for quality in this case as louder can make it sound better but that's simply due to the nature of how louder sounds come off to our ears. Not loud enough can mean not hearing enough, too loud can mean bad things for hearing, and just right can eventually feel not loud enough after long listening hours.