Mac mini: Setting internal speakers as Default (and making that setting stick)?

Just got a new (2020) Mac mini.

I want to use the built-in speakers as my Default audio out and the microphone on my iPhone ear buds as my Default audio in.

Here's the problem:

In System Preferences/Sound/Output, both the external ear buds and the internal Mac mini speakers show up as options. The external ear buds is at the top of the list.

When I select internal Mac mini speakers, they work fine when using the iPhone ear buds microphone and the audio in and the Mac mini internal speakers as the audio out. Everything's great.

But, after a shutdown and restart, the computer automatically selects the external iPhone ear buds for the audio out, rather than the internal Mac mini speakers (which is how it was set before the shutdown and is how I want it as my automatic default).

Is there a way I can make the Mac mini speakers as my Default and make that selection stick, unless I manually select another option?

I tried unplugging the iPhone ear buds after the computer was shut down, and then restarting the computer and checking to make sure the internal speakers selection was highlighted in the Preferences/Sound/Output. But, the instant I plugged the ear buds back in to the Mac mini, the selection in the Preferences/Sound/Output shifted from the internal speakers to the external ear buds.

I really don't want to have to use my ear buds for my audio output during my many phone calls during the day. I much prefer the Mac mini speakers for this use.

NOTE: This is cross-post that is also on the Catalina forum, but I've had no replies there, so thought I'd try here.

Thank you.

Cheers,

Bryan