You are absolutely right about these points. It's about $60 - $40 for a sound interface, $10 for an OTG cable, and ~$10 for the recording app. At that price, it's in the ballpark of the Tascam DR 05 - which is $100 new and ~$80 used.
Each have their pros and cons. I like the convenience having the app on the phone, but that's really the only convenience. You still have to carry the interface and the OTG cable - so it's not the most convenient way to go. While I haven't run into the charging issue (as my recordings have so far only been 1-2 hours), I can see running into it when I record for 3+ hours. Speaking of reliability, at least with Android I record with the screen on because Android will put the app to sleep when the screen's off. Admittedly, I need to test this more.
The dedicated sound recorder is ultimately more reliable and more professional as it's a dedicated recording tool. After playing with the DJ app, sound interface, and cable, I now lean towards the dedicated recorder as being a better solution - as it's only $25 more - especially if you want to use your phone while you're recording.
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