Audiobus: Use your music apps together.
What is Audiobus? — Audiobus is an award-winning music app for iPhone and iPad which lets you use your other music apps together. Chain effects on your favourite synth, run the output of apps or Audio Units into an app like GarageBand or Loopy, or select a different audio interface output for each app. Route MIDI between apps — drive a synth from a MIDI sequencer, or add an arpeggiator to your MIDI keyboard — or sync with your external MIDI gear. And control your entire setup from a MIDI controller.
Download on the App StoreAudiobus is the app that makes the rest of your setup better.
I’ve found a USB to 3.5mm dongle solution for iPad Pro 2018 owners (with a USB Hub)
Plugged this into my Aukey USB C hub and it actually works!
TROND External USB Audio Adapter Sound Card, with One 3.5mm Aux TRRS Jack for Integrated Audio Out & Microphone in https://www.amazon.com/dp/B07L56C28R/ref=cm_sw_r_cp_tai_chUfDbHPM6Q1V
Hey Apple! You can take away my headphone jack but you’ll never take away my dongle!!
Alternatively, get this
https://satechi.net/collections/usb-type-c/products/aluminum-type-c-mobile-pro-hub
And plug in this, if you need more USB ports.
Anker 4-Port USB 3.0 Ultra Slim Data Hub for Macbook, Mac Pro/mini, iMac, Surface Pro, XPS, Notebook PC, USB Flash Drives, Mobile HDD, and More https://www.amazon.com/dp/B00XMD7KPU/ref=cm_sw_r_cp_tai_5rUfDb9VHTNDC
Which I can also confirm works.
Obviously this post is for people who need multiple ports. If you don’t, just get this.
USB-C to 3.5 mm Headphone Jack Adapter
https://store.apple.com/xc/product/MU7E2AM/A
Hope this helps someone.