UPDATE: After various failed attempts, I think I have finally found the right approach. HAMVoIP using IAX to communicate between my remote device (phone, laptop, etc.) and the HAMVoIP server (Asterisk+Allstar). I am using a direct connection between client and server, so there was no need to register my node with Allstar. For the client software, I am using Zoiper5. At the moment I am only using a test connection in sort of an intercom mode. But, it works. Next step is to modify my USB sound-card to add COS (Carrier detection) and PTT support.
I had some fairly restrictive requirements which made this harder than it might have needed to be. First, I wanted to use a simple, inexpensive, single-board computer, like a Raspberry-Pi, at the remote radio site. This pretty much ruled out Zello, which only runs in Windows, or Wine on a i386 Linux board. I spend two days trying to get i386 emulation and Wine working on the Pi, and finally gave up. It just won't work on Debian/Raspian Buster.
The next step was Asterisk with Allstar. I first tried using a special Raspian (RasPBX) image that came with Asterisk and FreePBX. That installed, but getting Allstar (apt_rpt) to work was pretty much hopeless. There is almost nothing on the net on how to do that. And then, finally, I found HAMVoIP, this is a special setup of Arch Linux with Asterisk+Allstar installed, and a simple config system. That worked.
On the other end, I want to use my Macintosh as the client. There seem to have been a number of soft phones for OS-X around 10 years ago, but almost nothing now. And, I needed IAX support as well to talk to the HAMVoIP server. But, Zoiper5 solved that problem.
Anyway, now that I have a working core system, I will take the next step to actually controlling a radio. Stay tuned...