If this is going to be a permanent config (Ultrastudio + H6 always connected), then you might be able to handle this a bit better by creating an Aggregate Device.
I have not tested the following in Resolve, as I don't currently have a USB microphone or second audio device to hand. But in the past I have done something similar with other apps, like Reaper, and I'd be reasonably confident it should work in Resolve as well:
0. Close Resolve if open.
1. Open Audio MIDI Setup
2. Click the + in the bottom left, and choose Create Aggregate Device
Optionally, give it a name, by clicking twice on 'Aggregate Device' in the sidebar.
3. Click on the Aggregate Device, and in the right pane, tick both the UltraStudio and the H6 to add these to the aggregate.
4. Now set this new device as default output and input in Mac Sound Preferences or Audio MIDI Setup
5. You now have a soundcard with X inputs and Y outputs, where X = the number of inputs on the UltraStudio + the number of inputs on the H6, and Y being the same for outputs.
6. You'll need to work out which numbered inputs correspond to the one device and which to the other - you can see the layout in Audio MIDI Setup under the Aggregate Device
7. Open Resolve, and configure it to to listen to whichever input corresponds to the H6, and to output to whichever output(s) match the UltraStudio (via Preferences -> Video and Audio I/O -> Speaker Setup -> Manual)
8. I would expect that these settings would now persist, as Resolve thinks it's using a standard soundcard.
9. If it works, go back to Audio MIDI Setup and click the question mark in the right, choose "Set aggregate device settings in Audio MIDI Setup on Mac" and read about drift correction, to see if you need to enable this. I don't think it will be required when one device is for input only and the other for output only, but read about it and enable it if you think it might be.
I'd definitely give this a go and see if it will avoid having to set-up Resolve's audio on every restart.