There are several ways to stream but the sonicport is most likely the best. I have to believe your issue is likely the IOS version or the cable. I am coming down for the JKB Gypsy Sally shows and we can troubleshoot if you havent figured it out by then. I am leary of upgrading my IOS and breaking my ability to stream so I cant really test that as a possibility.
But you can:
Stream directly from the Mixpre-6 using a USB-A to lightning cable and an app like AUM. The negative aspect is that the app side then dets the bitrate for recording. i've tried this before, however, and it works.
The tascam would work, this should work:
https://www.amazon.com/gp/product/B07G9N111W/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1If you have an older iphone you can use a TRRS cable from the Mixpre-6 to the headset port of the iphone