You need a working analogue output, not necessarily an expensive DAC. If your phone has no headphone socket, an active USB-C dongle supplies the conversion and amplification. If your existing output already works well with your IEMs, an upgrade may solve nothing.
1. Does it connect properly?
A passive USB-C adapter relies on support from the phone. An active dongle contains a DAC and headphone amplifier. Check compatibility with your phone, especially if you also need a microphone or volume controls.
2. Is there enough clean volume?
Try a quiet recording at your normal listening level. If the output is near its limit, or clips on peaks, more usable voltage or current may help. First rule out EQ clipping and a poor ear-tip seal.
Impedance alone does not tell you how demanding an IEM is. Sensitivity matters too, and dB/mW cannot be compared directly with dB/V. Benchmark explains the distinction.
3. Is the output quiet and easy to control?
With sensitive IEMs, low hiss, gentle volume steps and good channel balance can matter more than maximum power. Start with low gain. More gain does not create more musical detail.
Buy for the problem you actually have. No headphone socket? Get a compatible dongle. Audible hiss? Look for a quieter output. Already clean, sufficiently loud and easy to adjust? Keep listening before spending more.
Related: Find the source of IEM hiss and DAC and USB-C guides.