Just check your needs and try to think about the next years. It would be a pitty to be at a similar point next year
Many of the small mixers (4 to 8 channels) provide very good audio quality and an USB stereo in-/out- connection to a computer. If you want to integrate the iPad as pro-quality audio-source or FX, I would recommend to use the digital Lightening interface only. Those iConnectivity audio/midi interfaces have the advantage that we can have computer and iPad digitally connected (audio and midi), and audio/midi from/to external devices handled by one single device too and everything at the same time.
You can, of course, connect the audio-mixer audio-outs (e.g. the master stereo outs, or sends) to your Zoom U24 and transmit the data to the computer via USB. What would not work on a PC, is to have an USB audio-mixer AND the Zoom U24 used simultaneously, because up today PC/Windows does not support more then one audio-interface. We can switch between different interfaces, but only have one in use. This includes all potential internal and external audio-interfaces. A Mac is different there.