The 9038SG3 can most certainly be used with an iPhone w/o an external power bank.
However, Apple's "MFi" program has made powering USB accessories from your iPhone needlessly complex for the end user.
What is required is a Lightning to Type-C cable with an Apple chip inside, which tells the iPhone that it is safe to supply power to the connected device.
This type of cable is available from a variety of brands, however, different cables (and Apple chips) will unlock different amounts of maximum allowed current. Exceed that limit and your iPhone will throw a "The connected USB accessory requires too much power" error message.
What's more, the same exact USB cable plugged into different iPhone models can unlock different amounts of current.
And as if that wasn't enough, recent reports from E1DA customers indicate that Apple may out of nowhere change these current limits with an iOS update.
The best chance at getting a compatible USB cable is this spreadsheet I created with the help of E1DA's customers:
https://docs.google.com/spreadsheets/d/1_Yqd7lpt3N2B0kVwzqzHoRmCTO7ZTe32p_xhjuGluh8/edit?usp=sharing
As for the volume control thing, that was an issues exclusive to the 9038S G1 and G2.
You see, Apple thought it unnecessary to give iOS and macOS software volume control, which means that if you plug an external DAC into your iPhone, then the only way to retain volume control through your phone's buttons is if the DAC supports UAC2 Hardware volume control.
The G1 and G2 did not support this feature and so the volume was locked at 100%.
The G3 on the other hand supports UAC2 volume commands, and as a result, controlling volume is seamless.
FYI, Windows supports both software volume and UAC2 hardware volume, so if your DAC doesn't have UAC2, it'll just use software. With a UAC2 DAC, Windows will automatically switch to that type of control w/o the user ever noticing what's happening under the hood.
Android on the other hand only supports software volume control, so it doesn't matter if your DAC understands UAC2 commands.